1. Type and Articular Bones
2. Ligaments
3. Relations
4. Actions/Muscles producing these actions
5. Applied Aspects

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HIP JOINT - 15 Marks (MBBS Level)

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1. TYPE AND ARTICULAR BONES

• Head of femur (spherical) - covered by hyaline cartilage except at the fovea capitis
• Acetabulum of the hip bone (os coxae) - formed by ilium (2/5), ischium (2/5), pubis (1/5); only the lunate surface is articular (covered by hyaline cartilage); broader superiorly where the greatest weight is transmitted

• Acetabular labrum: a fibrocartilaginous collar around the rim of the acetabulum that deepens the socket and grips the femoral head
• Transverse acetabular ligament: bridges the acetabular notch inferiorly, converting it into a foramen through which vessels and nerves pass

        ANTERIOR VIEW
     _______________
    |   ACETABULUM  |
    |  (Lunate surf)|
  __|_______________| ←── Acetabular labrum
 /    ○ Fovea        \
|   HEAD OF FEMUR    |
 \_______|__________/
         |
       NECK
         |
  -------+-------  ← Intertrochanteric line
  GT              LT
  (Greater      (Lesser
  trochanter)   trochanter)

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2. LIGAMENTS

a) Iliofemoral Ligament (Ligament of Bigelow)

• Strongest ligament in the body (inverted Y-shape)
• Apex: Anterior inferior iliac spine (AIIS) and acetabular rim
• Base: Along the intertrochanteric line (upper and lower bands are thicker than the central part - hence "Y" shape)
• Prevents hyperextension and lateral rotation

b) Pubofemoral Ligament

• Antero-inferior to the joint
• Base: Iliopubic eminence, superior rami of pubis, and obturator membrane
• Blends with the deep surface of the iliofemoral ligament
• Prevents excessive abduction and lateral rotation

c) Ischiofemoral Ligament

• Reinforces the posterior aspect of the capsule
• Medial attachment: Ischium, posteroinferior to acetabulum
• Lateral: Greater trochanter (deep to iliofemoral ligament)
• Prevents hyperextension and medial rotation

d) Ligament of Head of Femur (Ligamentum Teres)

• Flat triangular band inside the joint
• From: Fovea capitis (head of femur)
• To: Acetabular fossa, transverse acetabular ligament, and margins of acetabular notch
• Not mechanically important for stability; carries a branch of the obturator artery (supplies head of femur in children)

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3. RELATIONS

ANTERIOR:

• Iliopsoas muscle (separated from capsule by iliopectineal bursa)
• Pectineus, Rectus femoris
• Femoral nerve, artery, and vein (in the femoral triangle, lateral to medial: Nerve, Artery, Vein = NAV)

POSTERIOR:

• Sciatic nerve (most important posterior relation - can be injured in posterior dislocation)
• Piriformis, Obturator internus + gemelli
• Gluteus maximus (superficially)

SUPERIOR:

• Reflected head of rectus femoris
• Gluteus minimus

INFERIOR:

• Obturator externus tendon
• Transverse acetabular ligament

              SUPERIOR
         Gluteus minimus
              ↑
ANTERIOR                    POSTERIOR
Iliopsoas →  [HIP]  ← Sciatic nerve
Femoral NVB  [JOINT]   Piriformis
             ↓
           Obturator externus
              INFERIOR

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4. ACTIONS AND MUSCLES PRODUCING THEM

Key point (MBBS exam favourite): The Iliopsoas is the most powerful flexor of the hip. Gluteus maximus is the most powerful extensor (acts most powerfully when climbing stairs or rising from a chair). Gluteus medius abducts and prevents pelvic drop during walking - tested by Trendelenburg's test.

• Flexors: Femoral nerve (L2, L3, L4)
• Extensors: Inferior gluteal nerve (L5, S1, S2) for gluteus maximus; sciatic nerve for hamstrings
• Abductors: Superior gluteal nerve (L4, L5, S1)
• Adductors: Obturator nerve (L2, L3, L4)
• Lateral rotators: Nerve to piriformis, obturator nerve

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5. APPLIED ASPECTS

a) Dislocation of Hip Joint

• Posterior dislocation - most common (80-85%)
  • Mechanism: Forced flexion + adduction + internal rotation (dashboard injury)
  • Limb position: Flexed, adducted, internally rotated, shortened
  • Danger: Injury to sciatic nerve → foot drop
• Anterior dislocation - rare
  • Limb: Extended, abducted, externally rotated
  • Danger: Injury to femoral vessels and nerve

b) Fracture-Dislocation

• Avascular Necrosis (AVN) of femoral head: The femoral head gets its blood supply mainly from the medial circumflex femoral artery (branch of profunda femoris). Dislocation or neck-of-femur fracture disrupts this supply → AVN. The branch via ligamentum teres (obturator artery) is significant only in childhood.

c) Congenital Dislocation of Hip (CDH / DDH)

• More common in females (6:1)
• Acetabulum is shallow, femoral neck anteversion increased
• Ortolani test (click on abduction) and Barlow test (provocation test)
• Trendelenburg gait if bilateral

d) Trendelenburg's Sign

• Tests the abductor mechanism (gluteus medius/minimus + superior gluteal nerve + hip joint)
• Positive: When standing on the affected leg, the pelvis dips on the opposite side
• Causes: CDH, coxa vara, polio, fracture neck of femur, gluteus medius weakness, superior gluteal nerve injury

e) Osteoarthritis of Hip

• Most common joint disease affecting the hip in elderly
• Pain felt in groin, radiates to the knee (referred via obturator and femoral nerves)
• Thomas's test: To unmask fixed flexion deformity
• Managed by: NSAIDs, physiotherapy, and Total Hip Replacement (THR) in severe cases

f) Femoral Neck Fractures (subcapital)

• Blood supply disrupted → AVN and non-union
• Garden's classification (I-IV)
• Managed by: Internal fixation (Garden I-II) or hemiarthroplasty/THR (Garden III-IV) in elderly

g) Septic Arthritis

• In children, the hip is at risk from spread of proximal femoral osteomyelitis
• Emergency - must drain the joint early to prevent AVN

h) Haemarthrosis

• Blood can accumulate in the joint space - distension of capsule is most felt anteriorly (where capsule is thinnest)

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         HIP JOINT
    Ball & Socket (Synovial)
    ┌─────────────────────────┐
    │  HEAD femur + ACETABULUM│
    │  (lunate surface)       │
    │  Labrum deepens socket  │
    └─────────────────────────┘
         ↙           ↘
    LIGAMENTS       RELATIONS
  Ilio (strongest) Ant: Femoral NVB
  Pubo (abduction) Post: Sciatic N.
  Ischio (post)    Sup: Glu.minimus
  Lig.teres(intra) Inf: Obt.externus
         ↓
    MOVEMENTS
  Flex: Iliopsoas
  Ext:  Glu.max
  Abd:  Glu.med
  Add:  Adductors
  Lat.rot: Piriformis
         ↓
     APPLIED
  Post.disloc→Sciatic N.injury
  AVN→Blood supply disrupted
  CDH→Ortolani/Barlow test
  Trendelenburg→Glu.med weakness

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KNEE JOINT - 15 Marks (MBBS Level)

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1. TYPE AND ARTICULAR SURFACES

1. Tibiofemoral joint - between femoral condyles and tibial condyles (weight-bearing)
2. Patellofemoral joint - between posterior patella and patellar surface of femur (redirects pull of quadriceps)

• Medial meniscus: C-shaped, firmly attached to capsule and tibial collateral ligament (less mobile, more commonly torn)
• Lateral meniscus: More circular (O-shaped), NOT attached to fibular collateral ligament (more mobile, less commonly torn)
• Function: Deepen tibial surface, absorb shock, improve lubrication, fill gaps during complex knee movements

         ANTERIOR
    ________________
   |    Patellar    |
   |    lig. area   |
   |                |
   |  C-shaped   O-shaped|
   | [MEDIAL  ] [LATERAL]|
   | MENISCUS   MENISCUS |
   |     ACL  PCL       |
   |________________|
         POSTERIOR

M = more firmly fixed (more tears)
L = more mobile (fewer tears)

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2. LIGAMENTS

EXTRACAPSULAR LIGAMENTS:

• Continuation of quadriceps tendon below the patella
• Apex of patella → Tibial tuberosity
• Strongest anterior structure

• Flat, broad band on medial side
• Medial epicondyle of femur → Medial condyle + medial surface of tibia (below condyle)
• Firmly attached to the medial meniscus and joint capsule
• Prevents valgus (lateral) stress and lateral rotation
• Most commonly injured collateral ligament

• Cord-like (not attached to capsule or lateral meniscus)
• Lateral epicondyle of femur → Head of fibula
• Prevents varus (medial) stress

• Posterior aspect - expansion from semimembranosus tendon
• Prevents hyperextension

• Y-shaped, posterior capsule
• Arches over popliteus muscle

INTRACAPSULAR (CRUCIATE) LIGAMENTS:

• Tibial attachment: Anterior intercondylar area of tibia
• Femoral attachment: Posterior part of medial surface of lateral condyle of femur
• Direction: Passes upward, backward, and laterally
• Function: Prevents anterior displacement of tibia on femur (prevents forward sliding of tibia)
• Test: Anterior Drawer Test, Lachman Test

• Tibial attachment: Posterior intercondylar area of tibia
• Femoral attachment: Lateral surface of medial condyle of femur
• Direction: Passes upward, forward, and medially
• Stronger and tighter than ACL
• Function: Prevents posterior displacement of tibia on femur
• Test: Posterior Drawer Test (tibial sag sign)

Memory tip: ACL and PCL cross like an "X" in the intercondylar notch - ACL goes Anterior on tibia → Lateral femoral condyle; PCL goes Posterior on tibia → medial femoral condyle.

      FEMUR
    Med  |  Lat
     |   |   |
  PCL\  |  /ACL
      \ | /
       \|/
     Intercondylar notch
        |
    [TIBIA]
  Post.area  Ant.area
  (PCL)     (ACL)

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3. MOVEMENTS AND MUSCLES PRODUCING THEM

Rotation is only possible when the knee is flexed - in full extension the knee is "locked" and rotation is not possible.

Locking and Unlocking of Knee:

• The medial femoral condyle is longer than the lateral condyle
• As full extension approaches, the lateral condyle finishes its rolling first
• The femur then medially rotates on the tibia (or tibia laterally rotates on femur in open kinetic chain)
• All ligaments become taut → joint is "screwed home" and locked
• Requires NO muscle energy to maintain standing

• Popliteus muscle contracts → laterally rotates the femur (or medially rotates the tibia) → loosens the ligaments → unlocking occurs → flexion can begin

LOCKING:              UNLOCKING:
Knee extension          Popliteus contracts
↓                       ↓
Medial rotation         Lateral rotation of femur
of femur               (or medial rotation of tibia)
↓                       ↓
Ligaments taut          Ligaments loosened
↓                       ↓
Joint LOCKED            Flexion possible

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4. APPLIED ANATOMY

a) Unhappy Triad (O'Donoghue's Triad)

• Caused by: Valgus force + lateral rotation on a fixed, weight-bearing foot (common in football/rugby)
• Injuries: MCL + Medial meniscus + ACL torn simultaneously
• Patient unable to bear weight, rapid haemarthrosis
• MRI confirms; surgical repair required

b) Meniscal Tears

• Medial meniscus more commonly torn (attached to MCL and capsule - less mobile)
• Types: Vertical tear, Horizontal tear, Bucket handle tear (longitudinal - fragment displaces into intercondylar notch = joint locking)
• Symptoms: Pain on joint line, clicking, locking, giving way sensation
• Investigation: MRI (gold standard), Arthroscopy for repair

c) ACL Rupture

• Commonest serious knee ligament injury in sports
• Mechanism: Sudden change of direction/cutting, pivoting, landing
• Features: Sudden "pop" sound, rapid haemarthrosis, instability
• Tests: Anterior drawer test, Lachman test (more sensitive)
• Treatment: Physiotherapy in low-demand patients; ACL reconstruction (hamstring/patellar tendon graft) in athletes

d) PCL Injury

• Mechanism: Dashboard injury (knee forced backward when hitting dashboard), hyperextension
• Tibial sag sign / posterior drawer test positive
• Usually needs surgical repair

e) Housemaid's Knee (Prepatellar Bursitis)

• Inflammation of the prepatellar bursa (between patella and skin)
• Caused by: Prolonged kneeling (housemaids, carpet layers, plumbers)
• Features: Fluctuant swelling anterior to patella, knee movements painless
• Treatment: Aspiration, NSAIDs, avoid kneeling; excision for chronic cases

f) Osteoarthritis of Knee

• Commonest joint disease in elderly
• Medial compartment most affected → genu varum (bow legs)
• X-ray: Joint space narrowing, osteophytes, subchondral sclerosis
• Treatment: Weight loss, physiotherapy, NSAIDs; Total Knee Replacement (TKR) in severe cases

g) Locking of Knee

• True locking: Bucket-handle meniscal tear fragment blocks extension
• Springy block at ~30° of extension (cannot fully extend)
• Pseudo-locking: Painful hamstring spasm - gradual onset
• Treatment: Arthroscopic meniscectomy/repair

h) Bursae Around Knee (Enumeration)

1. Suprapatellar bursa - between femur and quadriceps (communicates with joint = largest extension of synovial cavity)
2. Prepatellar bursa - between patella and skin (Housemaid's knee if inflamed)
3. Infrapatellar bursa - superficial (between patellar ligament and skin) and deep (between patellar ligament and tibia) = "Clergyman's knee" if inflamed

i) Intracapsular Structures (Enumeration)

1. Anterior cruciate ligament (ACL)
2. Posterior cruciate ligament (PCL)
3. Medial meniscus
4. Lateral meniscus
5. Infrapatellar fat pad
6. Synovial membrane folds (alar folds, infrapatellar fold)
7. Transverse ligament of knee (connecting anterior horns of menisci)

         KNEE JOINT
     (Modified Hinge - Synovial)
    ┌────────────────────────────┐
    │  2 Tibiofemoral +          │
    │  1 Patellofemoral          │
    │  Menisci: Med(C) + Lat(O)  │
    └────────────────────────────┘
    Ligaments:
    Extracapsular:           Intracapsular:
    - Patellar (ant)         - ACL (ant draw)
    - MCL (valgus stress)    - PCL (post draw)
    - LCL (varus stress)
    - Obl./Arcuate (post)
    
    Movements:
    Flex: Hamstrings         Ext: Quadriceps
    Unlock: Popliteus (key muscle!)
    
    Applied:
    Unhappy triad→MCL+Med.men+ACL
    Housemaid's→Prepatellar bursitis
    Baker's cyst→Semimembranosus bursa
    Locked knee→Bucket handle tear

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📘 BIOCHEMISTRY IMPORTANT QUESTIONS - 15 MARKS EACH

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Q1. FORMATION, TRANSPORT, STORAGE & DETOXIFICATION OF AMMONIA. NOTE ON UREA CYCLE DISORDERS (15 Marks)

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A. SOURCES OF AMMONIA IN THE BODY

1. Transamination + Oxidative deamination of amino acids (major source)
2. Glutamine hydrolysis by glutaminase (in kidney and intestine)
3. Deamination of adenine/cytosine (nucleotide catabolism)
4. Bacterial action on intestinal proteins - amino acids → NH₃ absorbed via portal vein
5. Deamination of biogenic amines (histamine, serotonin etc.)
6. Hydrolysis of urea by urease of intestinal bacteria

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B. TRANSPORT OF AMMONIA

1. Glucose-Alanine Cycle (Cahill Cycle) - From MUSCLE

Muscle:
Pyruvate + NH₃ → ALANINE (via transamination)
                    ↓ (blood)
Liver:
Alanine → Pyruvate + NH₃ → Enters Urea Cycle
Pyruvate → Glucose (gluconeogenesis) → back to muscle

2. Glutamine Transport (from BRAIN and other tissues)

NH₃ + Glutamate → GLUTAMINE  [Glutamine synthetase]
                    ↓ blood to kidney/liver
                    ↓
GLUTAMINASE → Glutamate + NH₃ → Excreted as NH₄⁺ in urine
              (kidney) or enters urea cycle (liver)

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C. DETOXIFICATION OF AMMONIA - THE UREA CYCLE (Krebs-Henseleit Cycle)

Steps of Urea Cycle:

MITOCHONDRIA:
Step 1: NH₃ + CO₂ + 2ATP → CARBAMOYL PHOSPHATE
        [Enzyme: Carbamoyl Phosphate Synthetase-I (CPS-I)]
        [Activator: N-Acetyl Glutamate (NAG)]

Step 2: Carbamoyl Phosphate + Ornithine → CITRULLINE
        [Enzyme: Ornithine Transcarbamoylase (OTC)]
        [Citrulline exits to cytoplasm]

CYTOPLASM:
Step 3: Citrulline + Aspartate + ATP → ARGININOSUCCINATE
        [Enzyme: Argininosuccinate Synthetase (ASS)]
        [2nd nitrogen from aspartate enters here]

Step 4: Argininosuccinate → ARGININE + Fumarate
        [Enzyme: Argininosuccinate Lyase (ASL)]
        [Fumarate → TCA cycle]

Step 5: Arginine + H₂O → ORNITHINE + UREA
        [Enzyme: Arginase]
        [Ornithine re-enters mitochondria to restart cycle]

         MITOCHONDRIA            CYTOPLASM
         ┌──────────┐          ┌──────────────┐
NH₃+CO₂→[CPS-I]    │          │              │
         ↓          │          │  Citrulline  │
   Carbamoyl        │          │  + Aspartate │
   Phosphate        │    ──→   │  [ASS]↓      │
         + Ornithine│          │Argininosucct.│
   [OTC]↓           │          │  [ASL]↓      │
         Citrulline─┼──→──┐    │  Arginine    │
         └──────────┘     │    │  [Arginase]↓ │
                          └───→│  UREA + Orn  │
         Ornithine←────────────└──────────────┘

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D. OTHER DETOXIFICATION PATHWAYS

1. Renal excretion as NH₄⁺ - In kidney tubules, NH₃ + H⁺ → NH₄⁺ (excreted in acidic urine) - helps in acid-base balance
2. Glutamine synthesis - Storage/transport form
3. Asparagine synthesis - NH₃ added to aspartate
4. Glutamate synthesis - reductive amination of α-ketoglutarate

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E. HYPERAMMONEMIA - CLINICAL ASPECTS

• Depletes α-ketoglutarate (TCA cycle intermediate) → TCA cycle impaired → ↓ ATP in brain
• Excess glutamine in astrocytes → osmotic swelling → cerebral edema
• Alters neurotransmitter balance (GABA, glutamate)

• Nausea, vomiting, lethargy
• Confusion, asterixis (flapping tremor)
• Slurred speech, ataxia
• Stupor → Coma (hepatic encephalopathy)
• In neonates: poor feeding, seizures, respiratory alkalosis

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F. UREA CYCLE DISORDERS

• X-linked recessive (gene: Xp21.1)
• Males severely affected; females: variable (Lyon hypothesis)
• Key: ↑ Orotic acid in urine (carbamoyl phosphate diverts to pyrimidine synthesis)
• Diagnosis: Plasma amino acids, urine orotic acid, genetic testing

1. Restrict protein in diet (reduce nitrogen load)
2. Alternative pathway drugs:
   • Sodium benzoate → hippurate (excreted, carries N)
   • Sodium phenylbutyrate → phenylacetylglutamine (excreted, carries 2N)
3. Arginine/Citrulline supplementation (for OTC, ASS, ASL deficiencies)
4. Liver transplantation (curative - restores urea cycle enzymes)
5. Treat hyperammonemia: Lactulose, rifaximin (reduce gut NH₃), dialysis

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Q2. ENUMERATE LIVER FUNCTION TESTS (LFT) & THEIR INTERPRETATION (15 Marks)

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LIVER FUNCTION TESTS - CLASSIFICATION

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GROUP I: TESTS FOR HEPATOCELLULAR DAMAGE (Leak enzymes)

1. Serum Aminotransferases (Transaminases)

• AST:ALT ratio (De Ritis ratio):
  • <1: Viral hepatitis (ALT predominates - more liver-specific)

  • 2: Alcoholic liver disease (AST rises more; alcohol induces mitochondrial AST)

  • 3: Strongly suggests alcoholic hepatitis

• Very high AST/ALT (>1000 IU/L): Acute viral hepatitis, ischemic hepatitis, paracetamol toxicity

2. Serum Alkaline Phosphatase (ALP)

• Normal: 44-147 IU/L
• Elevated in: Cholestasis (biliary obstruction), bone disease (Paget's, bone mets), pregnancy
• Isoenzymes: Liver ALP (most relevant), bone ALP, intestinal ALP
• GGT (Gamma-glutamyl transferase) - rises with ALP in liver disease but not bone disease (differentiates)

3. Serum GGT (Gamma GT)

• Normal: 5-55 U/L (M), 5-38 (F)
• Very sensitive marker for alcoholic liver disease
• Elevated by enzyme-inducing drugs (phenobarbitone, phenytoin)
• Also elevated in non-alcoholic fatty liver (NAFLD)

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GROUP II: TESTS FOR SYNTHETIC FUNCTION

4. Serum Albumin

• Normal: 3.5-5.5 g/dL
• Synthesized exclusively by liver; t½ = 20 days
• Decreased in: Chronic liver disease (cirrhosis), malnutrition, nephrotic syndrome, malabsorption
• Low albumin → ↓ oncotic pressure → Ascites, edema

5. Prothrombin Time (PT) / INR

• Normal PT: 11-13.5 seconds; INR: <1.2
• Liver synthesizes clotting factors I (fibrinogen), II, V, VII, IX, X
• Factors II, VII, IX, X are vitamin K-dependent
• Prolonged PT = poor prognostic sign in acute liver failure
• Not corrected by vitamin K injection = intrinsic hepatocellular failure
• Corrected by vitamin K = cholestatic disease (fat-soluble vitamin K malabsorption)

6. Serum Cholesterol & Bile Acids

• Liver synthesizes cholesterol and converts it to bile acids
• Low cholesterol: Severe hepatic failure
• ↑ Bile acids: Sensitive marker of hepatic dysfunction

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GROUP III: TESTS FOR EXCRETORY FUNCTION (Bilirubin Metabolism)

7. Serum Bilirubin

RBC breakdown
↓
Haem → Biliverdin → BILIRUBIN (indirect/unconjugated)
                      ↓ (bound to albumin in blood)
                      LIVER (UDP-glucuronyl transferase)
                      ↓
                      BILIRUBIN GLUCURONIDE (direct/conjugated)
                      ↓
                      Bile → Intestine
                      ↓ (gut bacteria)
                      Urobilinogen → Stercobilin (stool - brown colour)
                      ↑
                      Urobilinogen (reabsorbed) → kidney → Urobilin in urine

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GROUP IV: MISCELLANEOUS TESTS

8. Serum Proteins (Total & Electrophoresis)

• Normal total protein: 6-8 g/dL
• Globulin = Total - Albumin (normal 2-3.5 g/dL)
• A:G ratio (normal: 1.5-2.5:1) - reversed in cirrhosis (↑globulins from immune stimulation)

9. Serum Ammonia

• Normal: 15-45 μmol/L
• Elevated in: Hepatic encephalopathy, urea cycle disorders, portosystemic shunts

10. Serum Ferritin & Iron Studies

• ↑ in Haemochromatosis

11. Serum α₁-Antitrypsin

• ↓ in α₁-antitrypsin deficiency (PAS-positive globules in hepatocytes)

12. Ceruloplasmin

• ↓ in Wilson's disease (<20 mg/dL)

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LFT PATTERNS - QUICK SUMMARY

HEPATOCELLULAR DAMAGE:    ↑↑↑ ALT/AST, ↑ Bilirubin (both), ↑ PT, ↓ Albumin
CHOLESTASIS (OBSTRUCTION): ↑↑↑ ALP, ↑↑ GGT, ↑ Direct Bilirubin, Normal ALT
ALCOHOLIC HEPATITIS:       AST:ALT > 2, ↑ GGT, MCV elevated
CHRONIC LIVER DISEASE:    ↓ Albumin, ↑ PT, ↑ Globulins, ↑ Bilirubin (both)

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Q3. DISORDERS ASSOCIATED WITH PURINE METABOLISM & CATABOLISM (15 Marks)

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PURINE CATABOLISM PATHWAY

Adenine nucleotides (AMP)          Guanine nucleotides (GMP)
    ↓ AMP deaminase                     ↓ 5'-nucleotidase
    IMP                                 Guanosine
    ↓ 5'-nucleotidase                   ↓ purine nucleoside phosphorylase (PNP)
    Inosine                             Guanine
    ↓ PNP                               ↓ Guanase
    Hypoxanthine                        Xanthine
    ↓ Xanthine oxidase (XO)             ↓ XO
    Xanthine ────────────────────────→ Uric Acid (EXCRETED)

• Males: 3.5-7.0 mg/dL
• Females: 2.5-6.0 mg/dL (oestrogen promotes uric acid excretion)

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DISORDER 1: GOUT (Hyperuricemia) - Most Clinically Important

• Primary Gout: Inborn error of metabolism or overproduction/underexcretion of uric acid (90%)
• Secondary Gout: Due to haematological malignancies, diuretics, chronic renal failure, etc.

1. Overproduction (10%): ↑ de novo purine synthesis; HGPRT deficiency; PRPP synthetase overactivity
2. Underexcretion (90%): Renal tubular defect; drugs (thiazide diuretics, aspirin, cyclosporin, alcohol)

• Rate-limiting step in purine synthesis de novo
• Overactivity → ↑ purine synthesis → ↑ uric acid

• Acute gouty arthritis - sudden onset, severe pain, red, hot, swollen joint (commonly 1st metatarsophalangeal joint - "podagra")
• Interval/intercritical gout - symptom-free between attacks
• Tophaceous gout - urate crystals deposited in soft tissues (ear pinna, olecranon bursa, Achilles tendon = tophi)
• Gouty nephropathy - urate crystals in renal tubules → renal failure
• Uric acid nephrolithiasis - radiolucent stones

• Serum uric acid > 7.0 mg/dL
• Joint fluid: Needle-shaped, negatively birefringent crystals under polarized light
• 24-hr urine uric acid: >600 mg = overproducer; <600 mg = underexcreter

• Acute attack: NSAIDs (indomethacin), Colchicine (blocks microtubule polymerization → inhibits neutrophil migration), Corticosteroids
• Long-term (prophylaxis):
  • Allopurinol (XO inhibitor → ↓ uric acid synthesis) - for overproducers
  • Febuxostat (newer XO inhibitor)
  • Probenecid (uricosuric - ↑ renal excretion) - for underexcreters
• Avoid: Purine-rich foods (red meat, seafood, alcohol/beer), thiazide diuretics, aspirin

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DISORDER 2: LESCH-NYHAN SYNDROME (LNS)

• HGPRT is needed for the salvage pathway of purine synthesis
• Without HGPRT: Hypoxanthine + Guanine cannot be salvaged back → excess hypoxanthine/xanthine → ↑↑↑ Uric acid
• Also: PRPP accumulates → ↑↑ de novo purine synthesis → even more uric acid

1. Hyperuricemia + Gout (uric acid 8-12 mg/dL)
2. Neurological features: Spastic cerebral palsy, choreoathetosis, dysarthria, intellectual disability
3. Self-mutilation (pathognomonic): Compulsive biting of lips, fingers, tongue

• ↑↑ Serum uric acid (often >10 mg/dL)
• ↑ Urine uric acid (>600 mg/day)
• Absent HGPRT activity in RBCs/fibroblasts
• X-linked (only males severely affected)

• Allopurinol (controls hyperuricemia but does NOT improve neurological symptoms)
• Physical restraints (self-mutilation)
• No curative treatment available

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DISORDER 3: ADENOSINE DEAMINASE (ADA) DEFICIENCY

• Without ADA: Adenosine and deoxyadenosine accumulate
• dATP accumulates in lymphocytes → inhibits ribonucleotide reductase → blocks DNA synthesis → lymphocyte apoptosis
• Result: Profound lymphopenia → no T cells, no B cells → Severe Combined Immunodeficiency (SCID)

• Recurrent severe infections from birth (bacterial, viral, fungal, opportunistic)
• Failure to thrive
• Pneumonia (PCP), oral candidiasis, chronic diarrhoea
• No tonsils/lymph nodes

• Profound lymphopenia (<500 cells/μL)
• Low IgG, IgA, IgM
• Absent ADA activity in RBCs
• ↑ Deoxyadenosine/adenosine in blood and urine

• PEG-ADA (pegylated bovine ADA) - enzyme replacement (first approved enzyme replacement therapy)
• HSCT (Haematopoietic Stem Cell Transplantation) - curative
• Gene therapy - first successful gene therapy trial (ADA-SCID was the model disease)

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DISORDER 4: PNP (Purine Nucleoside Phosphorylase) DEFICIENCY

• Autosomal recessive
• Accumulation of inosine and guanosine → dGTP accumulates in T cells → T-cell deficiency
• T-cell immunodeficiency (unlike ADA-SCID which affects both T & B cells)
• ↓ Uric acid (unlike gout - uric acid is low because PNP is needed to make xanthine/uric acid)

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SUMMARY TABLE - PURINE DISORDERS

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Q10. BIOCHEMICAL BASIS OF DIABETIC KETOACIDOSIS (DKA) (15 Marks)

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DEFINITION

1. Hyperglycemia (blood glucose >250 mg/dL)
2. Metabolic acidosis (pH <7.3, bicarbonate <15 mEq/L)
3. Ketonemia/Ketonuria (serum ketones positive)

---

BIOCHEMICAL BASIS / PATHOPHYSIOLOGY

Trigger: Absolute or Relative Insulin Deficiency + ↑ Counter-regulatory hormones (Glucagon, Cortisol, Catecholamines, GH)

INSULIN DEFICIENCY
        ↓
┌───────────────────────────────┐
│ ↓Glucose uptake by cells      │
│ ↑Glycogenolysis (liver)       │
│ ↑Gluconeogenesis (liver)      │ → HYPERGLYCEMIA
│ ↑ Glucagon/Cortisol           │
└───────────────────────────────┘
        ↓
HYPERGLYCEMIA → Osmotic diuresis → Polyuria, Polydipsia → Dehydration
        ↓ (>180 mg/dL renal threshold)
GLUCOSURIA

Ketogenesis:

INSULIN DEFICIENCY → ↑ HSL (Hormone-sensitive lipase) in ADIPOSE TISSUE
        ↓
LIPOLYSIS: Triglycerides → Glycerol + Free Fatty Acids (FFAs)
        ↓ (FFAs flood the liver)
BETA-OXIDATION: FFAs → Acetyl CoA (in excess)
        ↓ (OAA depleted due to gluconeogenesis)
EXCESS Acetyl CoA → KETOGENESIS
        ↓
ACETOACETATE ←→ BETA-HYDROXYBUTYRATE (major)
        ↓
      Acetone (volatile - fruity breath)

Metabolic Acidosis:

Ketone bodies (acetoacetate, beta-hydroxybutyrate) are WEAK ACIDS
↓ pH
↓ HCO₃⁻ (consumed in buffering)
→ HIGH ANION GAP metabolic acidosis
   Anion gap = Na⁺ - (Cl⁻ + HCO₃⁻) → elevated (>12 mEq/L)

---

CLINICAL FEATURES & BIOCHEMICAL CORRELATES

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LABORATORY FINDINGS

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TREATMENT OF DKA

1. Fluid replacement - Normal saline 0.9% (1L in first hour) - corrects dehydration
2. Insulin - Regular insulin IV infusion (0.1 U/kg/hr) - stops ketogenesis
3. Potassium replacement - Critical (insulin drives K⁺ into cells → dangerous hypokalemia)
4. Bicarbonate - Only if pH <6.9 (controversial)
5. Monitor: Blood glucose hourly; K⁺ every 2 hours; ABG, ketones
6. Switch to subcutaneous insulin when eating resumes and gap closed

---

DIAGRAM - DKA PATHOGENESIS

INSULIN DEFICIENCY
    ↓              ↓
LIVER              ADIPOSE TISSUE
↑Glycogenolysis    ↑Lipolysis
↑Gluconeogenesis   ↑FFAs
    ↓                  ↓
HYPERGLYCEMIA      FFAs → Liver
(>250 mg/dL)       ↓
Osmotic diuresis   β-OXIDATION
Dehydration        ↓
                   Acetyl CoA excess
                   ↓ (OAA busy in gluconeogenesis)
                   KETOGENESIS
                   ↓
               Acetoacetate
               β-Hydroxybutyrate → ACIDOSIS
               Acetone → Fruity breath

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Q11. COMPARE & CONTRAST HYPERTHYROIDISM & HYPOTHYROIDISM - TFT (15 Marks)

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THYROID HORMONE SYNTHESIS & REGULATION

Hypothalamus: TRH (Thyrotropin Releasing Hormone)
    ↓
Anterior Pituitary: TSH (Thyroid Stimulating Hormone)
    ↓
Thyroid Gland: T4 (Thyroxine) + T3 (Tri-iodothyronine)
    ↓ (negative feedback to hypothalamus & pituitary)
↑T4/T3 → ↓TRH, ↓TSH (negative feedback)
↓T4/T3 → ↑TRH, ↑TSH

---

COMPARISON TABLE

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TYPES OF HYPERTHYROIDISM WITH TFT PATTERN

TYPES OF HYPOTHYROIDISM WITH TFT PATTERN

---

APPLIED/CLINICAL POINTS

• Neonatal hypothyroidism (Cretinism): Screened by heel-prick TSH on day 3-5; if untreated → irreversible intellectual disability
• Myxedema coma: Severe hypothyroidism emergency; hyponatremia, hypothermia, bradycardia, coma; treat with IV T4 + hydrocortisone
• Thyroid storm: Severe hyperthyroidism crisis; fever, tachycardia, agitation; treat with PTU, propranolol, iodine (Lugol's)
• Amiodarone effect: Contains iodine; causes both hypo- and hyperthyroidism; amiodarone inhibits T4→T3 conversion → ↑FT4, ↓FT3, ↑rT3 even in euthyroid
• Pregnancy: Normal TSH range is lower; first trimester: TSH 0.1-2.5 mIU/L

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Q7. CALCIUM HOMEOSTASIS & FUNCTIONS OF CALCIUM (15 Marks)

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CALCIUM IN THE BODY

• Total body calcium: ~1000 g (99% in bone, 1% in ECF/ICF)
• Normal serum total calcium: 8.5 - 10.5 mg/dL (2.1-2.6 mmol/L)
• Normal ionized (free) calcium: 4.5 - 5.3 mg/dL (1.15-1.35 mmol/L) - physiologically active form
• Corrected calcium formula: Corrected Ca = Measured Ca + 0.8 × (4 - serum albumin) (Correct because ~40% calcium is protein-bound to albumin)

Total Calcium
    ├─ Protein-bound (40%): mainly albumin
    ├─ Ionized/free (50%): physiologically ACTIVE
    └─ Complexed with anions (10%): citrate, phosphate

---

REGULATORS OF CALCIUM HOMEOSTASIS

Three hormones control calcium:

HYPOCALCEMIA → stimulates →
    PARATHYROID GLAND → PTH release
        ↓ (actions of PTH):
        BONE: ↑ Osteoclast activity → ↑ Ca²⁺ release from bone
        KIDNEY: ↑ Ca²⁺ reabsorption (DCT); ↓ PO₄ reabsorption (proximal tubule → phosphaturia)
                ↑ 1α-hydroxylase → ↑ Calcitriol (active Vit D) formation
        INTESTINE (via Calcitriol): ↑ Ca²⁺ absorption

→ Serum Ca²⁺ rises → feedback inhibition of PTH

1. PTH (Parathyroid Hormone)

• 84 amino acid peptide
• Chief regulator of calcium homeostasis
• Secreted by chief cells of parathyroid gland
• Hypocalcemia → ↑PTH (negative feedback)
• Actions: ↑ bone resorption, ↑ renal Ca reabsorption, ↓ renal PO₄ reabsorption, ↑ calcitriol synthesis

2. Vitamin D (Calcitriol - 1,25-dihydroxycholecalciferol)

Sunlight on skin → 7-dehydrocholesterol → Cholecalciferol (Vit D3)
    ↓ (Liver: 25-hydroxylase)
25-hydroxycholecalciferol [major storage form; measured in serum]
    ↓ (Kidney: 1α-hydroxylase; stimulated by PTH, hypocalcemia, hypophosphatemia)
1,25-dihydroxycholecalciferol = CALCITRIOL (active form)

• Actions: ↑ Intestinal Ca²⁺ absorption (main action), ↑ intestinal PO₄ absorption, ↑ bone mineralization (alongside PTH in bone resorption at high doses)
• Deficiency → Rickets (children) / Osteomalacia (adults)

3. Calcitonin

• Secreted by C-cells (parafollicular cells) of thyroid
• Released in response to hypercalcemia
• Actions: ↓ osteoclast activity → ↓ bone resorption → ↓ serum Ca²⁺
• Less important in day-to-day homeostasis (thyroidectomized patients maintain normal Ca²⁺)

---

FUNCTIONS OF CALCIUM

1. Bone & Teeth: 99% of body Ca is in hydroxyapatite crystals [Ca₁₀(PO₄)₆(OH)₂] - structural role
2. Neuromuscular excitability: ↓Ca²⁺ → ↑neuromuscular excitability → tetany; ↑Ca²⁺ → ↓excitability
3. Muscle contraction: Ca²⁺ binds troponin C → allows actin-myosin interaction
4. Blood coagulation: Ca²⁺ (Factor IV) needed for activation of Factors VII, IX, X, prothrombin (all vitamin K-dependent); needed in thrombin generation
5. Second messenger: IP₃-Ca²⁺ signaling pathway (hormone action)
6. Enzyme activation: Salivary amylase, ATPase, lipase
7. Cell division: Spindle formation, exocytosis
8. Cardiac muscle: Ca²⁺ essential for cardiac action potential (plateau phase - L-type Ca channels)
9. Neurotransmitter release: Ca²⁺ triggers exocytosis of neurotransmitters at synapses

---

CALCIUM DISORDERS

Hypocalcemia (Ca < 8.5 mg/dL)

• Chvostek's sign: Tapping facial nerve → ipsilateral facial muscle twitching
• Trousseau's sign: Inflating BP cuff above systolic pressure for 3 min → carpal spasm (main d'accoucheur)
• Tetany, perioral paraesthesias, laryngospasm (stridor)
• Prolonged QT interval on ECG
• Seizures in severe cases

Hypercalcemia (Ca > 10.5 mg/dL)

• Bones: Osteitis fibrosa cystica, subperiosteal erosions, Brown tumours
• Stones: Renal calculi (calcium oxalate, calcium phosphate)
• Groans: Nausea, vomiting, constipation, peptic ulcer, pancreatitis
• Psychic Moans: Depression, confusion, psychosis, coma (if severe)

---

DIAGRAM - CALCIUM HOMEOSTASIS

                 HYPOCALCEMIA
                      ↓
           PARATHYROID GLAND
                      ↓ ↑PTH
         ┌────────────┼──────────────┐
         ↓            ↓              ↓
      BONE         KIDNEY          KIDNEY
   ↑ Osteoclasts  ↑ Ca reabsorb   ↑1α-hydroxylase
   ↑ Ca release   ↓ PO₄ reabsorb  ↓
                                  CALCITRIOL (Active Vit D)
                                       ↓
                                  INTESTINE
                                  ↑ Ca absorption
                                  ↑ PO₄ absorption
                      ↓
                 NORMOCALCEMIA → Negative feedback to PTH

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Q12. BLOOD GLUCOSE HOMEOSTASIS & METABOLISM DURING FASTING, FED & STARVATION (15 Marks)

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NORMAL BLOOD GLUCOSE RANGES

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A. FED STATE (Post-absorptive, 0-2 hrs after meal)

↑INSULIN actions:
LIVER:
  ↑ Glycolysis (↑glucokinase, ↑PFK-2)
  ↑ Glycogen synthesis (↑glycogen synthase)
  ↑ Fatty acid synthesis (acetyl CoA → fat)
  ↓ Gluconeogenesis
  ↓ Glycogenolysis

MUSCLE:
  ↑ Glucose uptake (GLUT-4 translocation)
  ↑ Glycogen synthesis
  ↑ Protein synthesis

ADIPOSE TISSUE:
  ↑ Glucose uptake (GLUT-4)
  ↑ Lipogenesis (↑LPL activity)
  ↓ Lipolysis (↓HSL)

• ~50% → oxidation for energy
• ~30% → glycogen (liver and muscle)
• ~20% → fat (triglycerides in adipose tissue)

---

B. FASTING STATE (Overnight, 8-12 hrs)

↑GLUCAGON actions:
LIVER:
  ↑ Glycogenolysis (↑glycogen phosphorylase)
  ↑ Gluconeogenesis (amino acids → glucose; glycerol → glucose)
  ↓ Glycolysis
  ↓ Glycogen synthesis

ADIPOSE TISSUE (via Catecholamines):
  ↑ Lipolysis → FFAs released → muscle/liver
  Glycerol → liver → gluconeogenesis

FUEL HIERARCHY in overnight fasting:
Hour 1-4: Liver glycogen provides glucose (hepatic glycogen ~75-100g = ~8 hrs supply)
Hour 4-16: Gluconeogenesis from amino acids (muscle protein breakdown) + glycerol

• Glycerol (from lipolysis)
• Lactate (from RBCs and exercising muscle - Cori cycle)
• Alanine (from muscle via alanine-glucose cycle)
• Pyruvate, Propionate

1. Pyruvate carboxylase → OAA (activated by acetyl CoA)
2. PEPCK → PEP
3. Fructose 1,6-bisphosphatase → F-6-P
4. Glucose-6-phosphatase → Glucose (liver only, not muscle)

---

C. STARVATION (>24-72 hours)

Phase 1 (0-24 hrs): Glycogen depletion

• Liver glycogen exhausted by ~24 hrs
• Gluconeogenesis becomes the sole source of glucose
• ↑↑ Glucagon, ↑ Cortisol, ↑ Catecholamines

Phase 2 (24 hrs - 1 week): Protein catabolism + Fatty acid mobilization

• Massive lipolysis → FFAs → liver → β-oxidation → Acetyl CoA
• Acetyl CoA → Ketone bodies (acetoacetate, β-hydroxybutyrate) - exported to peripheral tissues
• Muscle proteins broken down → amino acids → gluconeogenesis
• Blood glucose maintained at ~65-70 mg/dL (just enough for brain)

Phase 3 (>1 week): Ketone adaptation - BRAIN USES KETONES

• Brain adapts to use ketone bodies (crosses blood-brain barrier)
• Glucose requirement falls from 120g/day → 40g/day
• Protein sparing begins - less muscle breakdown
• Fat stores become primary fuel (can sustain weeks-months of starvation)

STARVATION TIMELINE:
0-4 hrs:    Fed → Liver glycogen mobilized
4-24 hrs:   Liver glycogen exhausted → Gluconeogenesis begins (from amino acids)
24-72 hrs:  ↑↑ Lipolysis → ↑FFAs → ↑Ketogenesis → Blood ketones rise
>72 hrs:    Brain switches to ketone bodies as fuel → ↓muscle catabolism
>2 weeks:   Metabolic adaptation - ketone bodies = ~70% of brain energy

---

INTERORGAN FUEL FLOW DIAGRAM

              STARVATION
                ↓
ADIPOSE TISSUE          MUSCLE
↑↑ Lipolysis            ↑Proteolysis
FFAs → Blood            Amino acids → Blood
Glycerol → Blood        Alanine/Glutamine
   ↓                         ↓
LIVER                   LIVER (gluconeogenesis)
β-oxidation             Glucose → Blood
↑ Acetyl CoA
↓
KETONE BODIES           BRAIN
(AcAc, βOHB) ─────→   (adapts to use ketones)
                ↓
HEART, MUSCLE          (Reduce glucose need)
(use ketones directly)

---

HORMONAL RESPONSES SUMMARY

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✅ QUESTIONS ANSWERED (15 Marks Each)

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📋 REMAINING QUESTIONS (from your list)

• Q4: Renal function tests & interpretation
• Q5: DNA & RNA structure, types of RNA
• Q6: Reference ranges of serum/ionized calcium + corrective calcium
• Q8: Copper-related disorders (Wilson's, Menkes)
• Q9: Significance of magnesium in clinical practice
• Q13: Osmotic acidemia (2 marks)
• Q14: Hyperuricemia vs Gout (2 marks)
• Q15: Zellweger syndrome (2 marks)
• Q16: Refsum disease (2 marks)
• Q17: Urea cycle disorder (4 marks)
• Q18: Azotemia vs Uremia (2 marks)
• Q19: eGFR vs Creatinine clearance (2 marks)
• Q20: Proteinuria (4 types)

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📘 BIOCHEMISTRY - REMAINING QUESTIONS

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Q4. ENUMERATE RENAL FUNCTION TESTS & THEIR INTERPRETATION (15 Marks)

---

INTRODUCTION

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GROUP I: TESTS OF GLOMERULAR FILTRATION

1. Serum Creatinine

• Normal: Males: 0.7-1.2 mg/dL | Females: 0.5-1.0 mg/dL
• Produced from creatine phosphate in muscle at a constant rate (proportional to muscle mass)
• Freely filtered, not reabsorbed, minimally secreted → good GFR marker
• Limitation: Serum creatinine does not rise until ~50% of GFR is lost (insensitive early marker)
• ↑ in: Renal failure, rhabdomyolysis, high meat diet, drugs (trimethoprim blocks tubular secretion of creatinine → spurious rise)

2. Blood Urea Nitrogen (BUN) / Blood Urea

• Normal BUN: 8-20 mg/dL | Blood Urea: 15-45 mg/dL
• Urea is end product of protein catabolism (urea cycle in liver)
• Filtered freely; 40-50% passively reabsorbed in tubules
• Less specific than creatinine (affected by protein intake, hydration, GI bleeding, catabolic state)

• Normal: 10:1 to 20:1
• >20:1 (Pre-renal): Dehydration, GI bleeding, high protein intake, cardiac failure → ↑urea reabsorption but creatinine less affected
• <10:1 (Intra-renal or Post-renal): ATN, liver disease (↓urea synthesis), low protein diet
• 15-20:1 with both elevated: Post-renal obstruction or intrinsic renal disease

3. Serum Uric Acid

• Normal: Males 3.5-7.0 mg/dL | Females 2.5-6.0 mg/dL
• Elevated in chronic renal failure (↓excretion), gout, myeloproliferative disorders
• Low in Fanconi syndrome (tubular loss), SIADH

4. Creatinine Clearance (CrCl)

• Formula:

CrCl (mL/min) = [Urine Cr (mg/dL) × Urine volume (mL/day)] / [Serum Cr (mg/dL) × 1440]

• Normal: Males: 97-137 mL/min | Females: 88-128 mL/min
• Overestimates GFR (because creatinine is also secreted by tubules)
• Requires accurate 24-hr urine collection

5. eGFR (Estimated GFR) - CKD-EPI or MDRD Formula

• Normal: >90 mL/min/1.73m²
• Calculated from serum creatinine + age + sex + race (no urine collection needed)
• Used for staging CKD:

6. Cystatin C

• Produced at constant rate by all nucleated cells; freely filtered, not secreted
• Better than creatinine (not affected by muscle mass, age, gender)
• Normal: 0.5-1.0 mg/L
• Rises earlier than creatinine in AKI

---

GROUP II: TESTS OF TUBULAR FUNCTION

7. Urine Specific Gravity & Osmolality

• Normal specific gravity: 1.003-1.030
• Normal urine osmolality: 50-1200 mOsm/kg (wide range = good concentrating ability)
• Isosthenuria (fixed SG 1.010): Tubular damage - cannot dilute or concentrate urine
• Urine:Plasma osmolality ratio:

  • 1.5: Pre-renal (concentrated urine = tubules intact)

  • <1.1: ATN (dilute urine = tubules damaged)

8. Urine Sodium

• Normal: 40-220 mEq/day
• Fractional Excretion of Sodium (FeNa):

FeNa (%) = [Urine Na × Plasma Cr] / [Plasma Na × Urine Cr] × 100

• FeNa <1%: Pre-renal AKI (tubules avidly reabsorb Na)
• FeNa >2%: Intrinsic renal (ATN) - tubules damaged, cannot reabsorb Na
• Exception: FeNa may be <1% in contrast nephropathy, myoglobinuria

9. Urine Protein

• Normal: <150 mg/day (or <30 mg albumin = microalbuminuria threshold)
• Microalbuminuria: 30-300 mg/day → earliest marker of diabetic nephropathy and hypertensive nephropathy
• Macroalbuminuria (overt proteinuria): >300 mg/day
• Nephrotic range: >3.5 g/day → edema, hypoalbuminemia, hyperlipidemia

1. Glomerular (most common): Large proteins leak - albumin, IgG (e.g., nephrotic syndrome)
2. Tubular: Low molecular weight proteins not reabsorbed (β₂-microglobulin, retinol binding protein) - Fanconi syndrome, heavy metal poisoning
3. Overflow: Protein overproduced beyond tubular capacity - Bence Jones protein (myeloma), myoglobin (rhabdomyolysis), hemoglobin
4. Functional: Orthostatic (postural) proteinuria - benign; protein appears on standing, not in early morning sample

10. Urine Glucose (Glycosuria)

• Normally absent (all glucose reabsorbed up to renal threshold ~180 mg/dL)
• Present in: Diabetes mellitus (hyperglycemia exceeds threshold), Renal glycosuria (Fanconi syndrome - ↓tubular reabsorption despite normal blood glucose)

11. Tubular Maximum (TmG) for Glucose

• Normal TmG = 375 mg/min (capacity to reabsorb glucose)

---

GROUP III: OTHER RENAL TESTS

12. Urinalysis (Routine & Microscopy)

• RBCs in urine (haematuria): >3 RBCs/HPF = significant
  • Dysmorphic RBCs / RBC casts → Glomerulonephritis
  • Isomorphic RBCs → Lower urinary tract (stone, cystitis)
• WBC casts: Pyelonephritis, interstitial nephritis
• Granular/Waxy casts: CKD
• Hyaline casts: Concentrated urine, normal
• Fatty casts: Nephrotic syndrome

13. Serum Electrolytes

• Sodium: 136-145 mEq/L (hyponatremia in SIADH/CKD; hypernatremia in dehydration)
• Potassium: 3.5-5.0 mEq/L (hyperkalemia in CKD/AKI → lethal arrhythmias)
• Bicarbonate: 22-26 mEq/L (↓ in renal tubular acidosis, CKD)

14. Urine Acidification Test (RTA workup)

• Ammonium chloride loading test
• In RTA Type 1: Urine pH cannot fall below 5.5 despite systemic acidosis

15. Water Deprivation Test (AVP test)

• For Diabetes Insipidus vs SIADH diagnosis
• Compares urine osmolality before and after water deprivation and after desmopressin (DDAVP)

---

RFT PATTERN INTERPRETATION TABLE

           Pre-renal     Intrinsic (ATN)    Post-renal
BUN:Cr      >20:1          10-15:1           >20:1 initially
Urine SG    >1.020         ~1.010 (fixed)    Variable
Urine Na    <20 mEq/L      >40 mEq/L         Variable
FeNa        <1%            >2%               Variable
U:P Osmol   >1.5           ~1.0              Variable
Response    Fluids ↑GFR    No response       Relieve obstruction

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Q5. DNA & RNA STRUCTURE - TYPES OF RNA & THEIR ROLES (15 Marks)

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A. DNA STRUCTURE

Double Helix Model (Watson & Crick, 1953)

• Deoxyribose sugar (2'-deoxyribose)
• Phosphate groups
• Nitrogenous bases:
  • Purines: Adenine (A), Guanine (G)
  • Pyrimidines: Thymine (T), Cytosine (C)

• A pairs with T (2 hydrogen bonds)
• G pairs with C (3 hydrogen bonds)
• A + G = C + T (purines = pyrimidines)
• A:T ratio = 1; G:C ratio = 1

5'────────────────────────3'  (sense/coding strand)
|  Phosphodiester backbone  |
|  Base pairs (rungs)       |
|  A═T  (2 H-bonds)         |
|  G≡C  (3 H-bonds)         |
3'────────────────────────5'  (antisense/template strand)

• Right-handed double helix (B-form, most common in cells)
• Helix diameter: 20 Å (2 nm)
• Base pair spacing: 3.4 Å (0.34 nm)
• One complete turn: 34 Å = 10 base pairs (pitch = 34Å)
• Antiparallel strands (one 5'→3', other 3'→5')
• Major groove and minor groove (proteins bind major groove for gene regulation)

• A-DNA: Right-handed, broader, dry conditions
• Z-DNA: Left-handed, found in GC-rich regions; may play role in gene regulation

DNA → Nucleosome (DNA + 8 histones = H2A, H2B, H3, H4 × 2 each; 147 bp wraps)
→ Solenoid → Chromatin fibre → Looped domains → Chromosome
Histone H1: Linker histone (between nucleosomes)

---

B. RNA STRUCTURE

---

C. TYPES OF RNA & THEIR ROLES

1. mRNA (Messenger RNA) - ~5% of total RNA

5' Cap (7-methyl guanosine) ─── 5'UTR ─── AUG (start codon) ─── Coding sequence 
─── Stop codon ─── 3'UTR ─── Poly-A tail (200-250 adenines)

• 5' Cap: Protects from exonucleases; required for ribosome binding (translation initiation)
• Poly-A tail: Added post-transcriptionally by poly-A polymerase; protects from degradation; aids nuclear export
• AUG (methionine): Universal start codon
• Stop codons: UAA, UAG, UGA ("UAA = U Are Away; UAG = U Are Gone; UGA = U Go Away")

DNA → Pre-mRNA (hnRNA - heterogeneous nuclear RNA)
↓ RNA processing:
1. 5' capping
2. Polyadenylation (3' poly-A tail addition)
3. SPLICING: Introns removed, Exons joined (by SPLICEOSOME - snRNPs)
↓
Mature mRNA → exported to cytoplasm → Translation

---

2. rRNA (Ribosomal RNA) - ~80% of total RNA (most abundant)

• Structural component of ribosomes
• Catalytic activity (ribosomal RNA is a ribozyme - the peptidyl transferase activity resides in 23S/28S rRNA, NOT in ribosomal proteins)

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3. tRNA (Transfer RNA) - ~15% of total RNA

           Anticodon loop
           (3 nucleotides complementary to mRNA codon)
               ___
              /   \
    ─────────      ─────────────── 3' CCA-OH (Amino acid attachment site)
    |        \    /                 (AA added by aminoacyl-tRNA synthetase)
    |     D-loop  TψC loop
    |
DHU arm (dihydrouridine loop - recognises synthetase)

• ~73-93 nucleotides
• Cloverleaf in 2D; L-shaped in 3D
• 3' end always: CCA-OH (universal; added post-transcriptionally)
• Anticodon loop: Complementary to mRNA codon (antiparallel, anticodon)
• Contains unusual modified bases: pseudouridine (ψ), dihydrouridine, inosine
• Wobble hypothesis (Crick): The 3rd base of codon can pair loosely with 1st base of anticodon → 1 tRNA can recognize multiple codons (explains why 64 codons need only ~45 tRNA molecules)
• Aminoacyl-tRNA synthetase: Charges tRNA with correct amino acid (uses ATP; "Second genetic code" - specific recognition of tRNA and amino acid)

---

4. snRNA (Small Nuclear RNA)

• Found in the nucleus; part of snRNPs (snurps)
• Role: Component of the spliceosome - catalyzes removal of introns from pre-mRNA
• Types: U1, U2, U4, U5, U6 snRNA
• Clinical: Anti-Sm antibodies (against snRNPs) are specific for SLE (Systemic Lupus Erythematosus)

---

5. snoRNA (Small Nucleolar RNA)

• Located in the nucleolus
• Role: Guides chemical modification (methylation, pseudouridylation) of rRNA and tRNA

---

6. miRNA (Micro RNA) & siRNA (Small Interfering RNA) - ~21-25 nucleotides

• Encoded in genome; processed by DROSHA (nucleus) then DICER (cytoplasm)
• Incorporated into RISC (RNA-Induced Silencing Complex)
• Binds 3'UTR of mRNA → translational repression or mRNA degradation
• Role: Gene expression regulation; tissue differentiation; oncogenes/tumour suppressors
• Clinical: miRNAs as biomarkers of cancer; miRNA-based therapeutics

• Exogenous (e.g., viral dsRNA) processed by DICER
• Leads to specific mRNA cleavage and degradation
• Role: RNA interference (RNAi) - anti-viral defense
• Clinical: siRNA drugs approved (e.g., Patisiran for TTR amyloidosis - first FDA-approved siRNA drug 2018)

---

7. lncRNA (Long Non-coding RNA)

• 200 nucleotides; do not code for protein

• Role: Chromatin remodelling, transcriptional regulation, X-chromosome inactivation (XIST lncRNA)

---

8. Ribozymes (Catalytic RNA)

• RNA molecules with enzymatic activity
• Examples: Self-splicing Group I and II introns; ribonuclease P; peptidyl transferase (rRNA)

---

RNA TYPES SUMMARY TABLE

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Q6. REFERENCE RANGES - SERUM CALCIUM, IONIZED CALCIUM, CORRECTIVE CALCIUM & IMPORTANCE (Short Note)

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REFERENCE RANGES

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DISTRIBUTION OF CALCIUM IN BLOOD

TOTAL SERUM CALCIUM (8.5-10.5 mg/dL)
        │
        ├─── Protein-bound (40%) ─── Mainly albumin (80%), globulins (20%)
        │    [NOT physiologically active]
        │
        ├─── Ionized/free (50%) ─── PHYSIOLOGICALLY ACTIVE
        │    [Measured separately; gold standard]
        │
        └─── Complexed (10%) ─── With citrate, phosphate, sulfate
             [Not ionized but not protein-bound]

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CORRECTIVE (ADJUSTED) CALCIUM FORMULA

Corrected Ca (mg/dL) = Measured Ca (mg/dL) + 0.8 × [4 - Serum Albumin (g/dL)]

• Patient: Ca = 8.0 mg/dL, Albumin = 2.0 g/dL (low in malnutrition/liver disease)
• Corrected Ca = 8.0 + 0.8 × (4 - 2.0) = 8.0 + 1.6 = 9.6 mg/dL → NORMAL
• Without correction: Would falsely diagnose hypocalcemia

• ICU patients on blood products (citrate binds Ca)
• Alkalosis (↑pH → Ca²⁺ binds more to albumin → ↓ionized Ca → tetany with normal total Ca)
• Acidosis (↓pH → Ca²⁺ released from albumin → ↑ionized Ca → protects against tetany)
• During major surgery, transfusions, cardiopulmonary bypass

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Q8. COPPER-RELATED DISORDERS (Short Note - 4/5 Marks)

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COPPER METABOLISM

• Dietary copper: 1-3 mg/day
• Absorbed in small intestine (via copper transporter CTR1)
• Transported in blood bound to ceruloplasmin (90-95%) - made in liver; or albumin/transcuprein
• Stored in liver
• Excreted mainly in bile (major route); small amount in urine
• Normal serum copper: 70-150 μg/dL
• Normal ceruloplasmin: 20-60 mg/dL
• Normal 24-hr urine copper: <50 μg/day

• Cytochrome c oxidase (electron transport), Superoxide dismutase (antioxidant), Ceruloplasmin (ferroxidase), Tyrosinase (melanin synthesis), Dopamine β-hydroxylase, Lysyl oxidase (collagen/elastin cross-linking)

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DISORDER 1: WILSON'S DISEASE (Hepatolenticular Degeneration)

Defective ATP7B → Cannot incorporate Cu into ceruloplasmin → Low ceruloplasmin
                  Cannot excrete Cu into bile
                       ↓
              Cu accumulates in LIVER first → then overflows into blood (free Cu)
                       ↓
              Deposits in: Liver, Brain (lenticular nucleus), Cornea, Kidney, RBCs

• ↓ Serum ceruloplasmin (<20 mg/dL) - most useful screening test
• ↑ 24-hr urine copper (>100 μg/day; >250 μg/day diagnostic)
• ↑ Free (non-ceruloplasmin) serum copper
• Liver copper >250 μg/g dry weight (biopsy - gold standard)
• LFT derangement
• KF rings on slit-lamp

• D-Penicillamine (copper chelator - first line; mobilizes tissue copper → urine) - SE: Lupus-like reaction, nephropathy
• Trientine (alternative chelator; fewer side effects)
• Zinc acetate (maintenance; blocks intestinal copper absorption - induces metallothionein in enterocytes)
• Tetrathiomolybdate (emergency, acute liver failure)
• Liver transplantation - curative (restores normal ATP7B)
• Diet: Avoid copper-rich foods (liver, shellfish, nuts, chocolate)

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DISORDER 2: MENKES DISEASE (Kinky/Steely Hair Disease)

• Defective ATP7A → Copper absorbed by intestinal cells but CANNOT be transported out
• Copper trapped in enterocytes → cannot reach liver, blood, or tissues
• Result: Systemic copper deficiency (despite increased total body copper in gut)

• Normal at birth (maternal copper crosses placenta)
• Onset: 2-3 months
• Kinky/steely hair (pili torti - twisted, sparse, depigmented hair) - hallmark
• Neurodegeneration (seizures, hypotonia, intellectual disability)
• Connective tissue abnormalities (loose skin, joint laxity) - ↓lysyl oxidase → defective collagen/elastin
• Arterial aneurysms (tortuous, ectatic arteries)
• Hypothermia, failure to thrive

• ↓ Serum copper (<50 μg/dL)
• ↓ Ceruloplasmin
• ↑ Copper in intestinal biopsy/fibroblasts

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COMPARISON TABLE

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Q9. SIGNIFICANCE OF MAGNESIUM IN CLINICAL PRACTICE (Short Note)

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MAGNESIUM - BASICS

• 4th most abundant cation in body; 2nd most abundant intracellular cation (after K⁺)
• Total body Mg: ~24g; 60% in bone, 39% intracellular (muscle), 1% in ECF
• Normal serum Mg: 1.5-2.5 mEq/L (0.75-1.25 mmol/L)
• Half protein-bound (to albumin); half free (ionized)
• Regulated by kidney (primary) and intestine (absorption)

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BIOCHEMICAL ROLES

1. Cofactor for >300 enzymes - especially all ATP-dependent reactions (Mg²⁺-ATP complex is actual substrate)
2. DNA/RNA synthesis: Mg²⁺ stabilizes phosphate groups; essential for DNA/RNA polymerases
3. Protein synthesis: Required by ribosomes (Mg²⁺ holds 30S and 50S/40S and 60S together)
4. Glycolysis and gluconeogenesis (hexokinase, PFK, enolase - all Mg²⁺ dependent)
5. Na⁺/K⁺-ATPase: Mg²⁺ essential → ↓Mg²⁺ → ↓Na/K pump → hypokalemia (refractory)
6. Voltage-gated Ca²⁺ channels: Mg²⁺ acts as physiological antagonist of calcium → "Natural calcium channel blocker"
7. Neuromuscular transmission: Mg²⁺ inhibits ACh release at motor end plate
8. Cardiac conduction: Stabilizes myocardial membrane; anti-arrhythmic

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HYPOMAGNESEMIA (Serum Mg <1.5 mEq/L)

• Malabsorption, alcoholism, diarrhoea, malnutrition
• Loop/thiazide diuretics (renal loss)
• DKA treatment (insulin drives Mg into cells)
• Aminoglycoside/Cisplatin/Amphotericin B nephrotoxicity
• Refeeding syndrome

• Neuromuscular: Tetany, tremor, carpopedal spasm (mimics hypocalcemia)
• Cardiac: Ventricular tachycardia, Torsades de Pointes (polymorphic VT - key)
• Refractory Hypokalemia: ↓Mg → ↓Na/K-ATPase → K⁺ cannot be retained → cannot correct K⁺ without fixing Mg
• Refractory Hypocalcemia: ↓Mg → ↓PTH secretion AND ↓end-organ PTH response → hypocalcemia
• Hyponatremia, personality changes

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HYPERMAGNESEMIA (Serum Mg >2.5 mEq/L)

Mg 4-5 mEq/L:  Nausea, flushing, ↓DTRs (deep tendon reflexes lost first)
Mg 5-7 mEq/L:  Drowsiness, ECG changes (↑PR, wide QRS)
Mg 7-10 mEq/L: Muscle paralysis, respiratory depression
Mg >12 mEq/L:  Cardiac arrest

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KEY CLINICAL USES OF MAGNESIUM THERAPY

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Q13. OSMOTIC ACIDEMIA (2 Marks)

• Toxic alcohols: Methanol, ethylene glycol, isopropanol
• Mannitol (exogenous)
• Ketoacidosis (acetone)
• Lactic acidosis

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Q14. HYPERURICEMIA vs GOUT (2 Marks)

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Q15. ZELLWEGER SYNDROME (2 Marks)

• Cannot perform very long chain fatty acid (VLCFA) β-oxidation (occurs in peroxisomes)
• Cannot synthesize plasmalogens (important ether phospholipids in myelin and heart)
• Cannot synthesize bile acids (peroxisomal step)
• Accumulation of VLCFAs (C26:0, C24:0) in blood - diagnostic marker

• Characteristic facies: High forehead, flat face, large fontanelle
• Profound hypotonia ("floppy baby")
• Seizures, intellectual disability
• Liver disease (hepatomegaly, cirrhosis)
• Renal cortical cysts
• Retinal dystrophy, hearing loss
• Most die within 1 year

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Q16. REFSUM DISEASE (2 Marks)

• Phytanic acid (a branched-chain fatty acid from chlorophyll in dairy, ruminant fat, and certain fish) cannot be degraded
• Phytanic acid is too branched for normal β-oxidation; requires α-oxidation first in peroxisomes
• Phytanic acid accumulates in blood, tissues, and nerves

1. Retinitis pigmentosa → night blindness → tunnel vision → blindness
2. Peripheral neuropathy → weakness, sensory loss
3. Cerebellar ataxia → gait disturbance
4. Elevated CSF protein (with normal cells)

• Dietary restriction of phytanic acid (avoid dairy fat, ruminant fat, certain fish like tuna, cod, haddock) - reduces accumulation and slows progression
• Plasmapheresis/LDL apheresis (in acute crisis to rapidly reduce phytanic acid)
• Treatment is effective if started early

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Q17. UREA CYCLE DISORDER (4 Marks)

Step 1: NH₃ + CO₂ → Carbamoyl-P  [CPS-I deficiency → ↑NH₃, normal orotic acid]
Step 2: Carbamoyl-P + Ornithine → Citrulline  [OTC deficiency → most common, X-linked, ↑orotic acid]
Step 3: Citrulline + Aspartate → Argininosuccinate  [Citrullinemia → ↑↑citrulline]
Step 4: Argininosuccinate → Arginine + Fumarate  [Argininosuccinic aciduria]
Step 5: Arginine → Urea + Ornithine  [Argininemia → ↑arginine, spasticity]

• Neonatal: Poor feeding, vomiting, lethargy, seizures, coma within days of birth
• Late-onset: Episodic encephalopathy triggered by high-protein meal, illness, surgery
• ↑ Blood ammonia, ↑ Glutamine, ↓ BUN (paradoxically low because urea not being made)

1. Protein restriction (0.5-1.5 g/kg/day)
2. Nitrogen scavengers: Sodium benzoate + Sodium phenylbutyrate (Ravicti®)
3. Arginine/Citrulline supplementation (for mid-cycle defects)
4. Liver transplant (curative)

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Q18. AZOTEMIA vs UREMIA (2 Marks)

• Pre-renal: ↓ Renal perfusion (dehydration, heart failure); BUN:Cr >20:1; responds to fluids
• Renal (intrinsic): ATN, GN, interstitial nephritis; BUN:Cr 10-15:1
• Post-renal: Obstruction (BPH, stone, tumour); treat obstruction

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Q19. eGFR vs CREATININE CLEARANCE (2 Marks)

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Q20. PROTEINURIA - 4 TYPES (4 Marks)

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TYPE 1: GLOMERULAR PROTEINURIA

• Nephrotic syndrome (minimal change disease, membranous nephropathy, focal segmental glomerulosclerosis)
• Diabetic nephropathy
• Lupus nephritis
• Amyloidosis

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TYPE 2: TUBULAR PROTEINURIA

• Fanconi syndrome (proximal tubular dysfunction - idiopathic, galactosemia, Wilson's, heavy metal poisoning, tenofovir toxicity)
• Interstitial nephritis
• Renal tubular acidosis
• Cadmium/lead/mercury poisoning
• Multiple myeloma with light chain cast nephropathy

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TYPE 3: OVERFLOW PROTEINURIA

• Multiple myeloma: Bence Jones proteins (free immunoglobulin light chains, κ or λ) - detected by urine immunoelectrophoresis (not dipstick! - dipstick detects albumin only, NOT Bence Jones)
• Myoglobinuria: Myoglobin (17 kDa) - in rhabdomyolysis (crush injury, statin toxicity, extreme exercise) → dark brown urine, ↑CK, AKI
• Hemoglobinuria: Free Hb - in intravascular hemolysis (PNH, G6PD deficiency, transfusion reaction) → dark red/brown urine
• Lysozymuria: In acute monocytic leukemia

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TYPE 4: FUNCTIONAL/PHYSIOLOGICAL PROTEINURIA

• Protein in urine when standing but absent when lying down (early morning sample protein-free)
• Mechanism: ↑renal venous pressure on standing → transient ↑protein leak
• Benign; common in adolescents (2-5%); no treatment needed
• Diagnosis: Split urine sample - recumbent (night) vs ambulatory (day); protein only in daytime sample

• Transient during high fever; resolves when fever resolves
• Mechanism: Cytokine-mediated transient ↑glomerular permeability

• Vigorous exercise → ↑protein in urine; resolves with rest
• Benign; no investigation needed if resolves

• ↑ renal venous pressure → transient proteinuria; resolves with treatment

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SUMMARY TABLE - PROTEINURIA TYPES

TYPE          PROTEIN          AMOUNT      KEY CAUSE          DIPSTICK
─────────────────────────────────────────────────────────────────────
Glomerular    Albumin, IgG     >1-3.5 g    Nephrotic synd.   Positive
Tubular       β₂-microglobulin <2 g        Fanconi syndrome  Mildly +ve
Overflow      BJP, Myoglobin   Variable    Myeloma, rhabdo.  Often -ve!
Functional    Albumin (mild)   <1 g        Exercise, fever   Trace/+

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