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📚 Biochemistry Paper 2 - Complete Exam Guide

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PRIORITY ORDER (by marks):

• 🔴 13 marks each: Protein & AA Metabolism, Molecular Biology, Molecular Biology Techniques, Heme Metabolism, Organ Function Tests
• 🟠 10 marks each: Nucleotide Metabolism, Biochemistry of Cancer
• 🟡 6 marks each: Protein Chemistry, Nucleic Acid Chemistry, Free Radicals & Antioxidants
• 🟢 5 marks each: Plasma Proteins, Immunology, Clinical Chemistry
• 🔵 3 marks: Xenobiotics & Detoxification

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🔴 TOPIC 1: Protein & Amino Acid Metabolism (13 marks)

Most Important Questions:

• Urea cycle occurs in liver (both mitochondria + cytoplasm)
• Converts toxic NH₃ → urea (excreted in urine)
• Steps:
  1. NH₃ + CO₂ + 2ATP → Carbamoyl phosphate (enzyme: CPS-I, in mitochondria)
  2. Carbamoyl phosphate + Ornithine → Citrulline (OTC enzyme)
  3. Citrulline → Argininosuccinate (+ Aspartate, ATP)
  4. Argininosuccinate → Arginine + Fumarate
  5. Arginine → Ornithine + Urea (arginase)
• Disorders: Hyperammonemia - ornithine transcarbamylase deficiency (most common, X-linked)

• Deficiency of phenylalanine hydroxylase (PAH)
• Phenylalanine accumulates → phenylpyruvate, phenyllactate in urine
• Features: intellectual disability, fair skin/hair/eyes, musty odor, seizures
• Treatment: Low-phenylalanine diet, tetrahydrobiopterin (BH4)
• Screening: Guthrie test (heel-prick blood spot)

• Transamination: Transfer of -NH₂ group from AA to α-keto acid (enzyme: transaminases/aminotransferases, coenzyme: PLP/Pyridoxal phosphate)
  • ALT: Alanine + α-KG ↔ Pyruvate + Glutamate
  • AST: Aspartate + α-KG ↔ Oxaloacetate + Glutamate
• Oxidative deamination: Glutamate → α-KG + NH₃ (enzyme: Glutamate dehydrogenase, GDH)

• Deficiency of branched-chain α-keto acid dehydrogenase
• Branched chain AAs accumulate: Leucine, Isoleucine, Valine
• Features: Sweet maple syrup odor of urine, encephalopathy, death if untreated
• Treatment: Diet restricted in branched-chain AAs

• SAM (S-Adenosylmethionine) = universal methyl donor
• Homocysteine → Methionine (requires folate + Vit B12)
• Homocystinuria: CBS enzyme deficiency → homocysteine accumulates → thromboembolism, Marfanoid habitus, lens dislocation, intellectual disability

• Tyrosine → DOPA → Dopamine → Norepinephrine → Epinephrine
• Rate-limiting enzyme: Tyrosine hydroxylase
• Degradation by MAO (monoamine oxidase) and COMT

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🔴 TOPIC 2: Molecular Biology (13 marks)

Most Important Questions:

• Semi-conservative replication (Watson-Crick)
• Key enzymes:

  Enzyme	Function
  Helicase	Unwinds double helix
  Primase	Synthesizes RNA primer
  DNA Pol III	Main replicating enzyme (prokaryotes)
  DNA Pol I	Removes primers, fills gaps
  DNA Ligase	Joins Okazaki fragments
  Topoisomerase	Relieves torsional stress

• Leading strand: continuous synthesis
• Lagging strand: discontinuous (Okazaki fragments)

• Template strand = antisense strand; coding strand = sense strand
• RNA Polymerase (no primer needed)
• 3 types of RNA:
  • mRNA - carries genetic info
  • tRNA - anticodon, carries AA (clover-leaf structure)
  • rRNA - structural component of ribosome (most abundant)
• Post-transcriptional modifications of mRNA: 5' capping, 3' poly-A tail, splicing of introns

• Codons: triplet code; AUG = start codon (Met)
• Stop codons: UAA, UAG, UGA
• Ribosomes: A site (aminoacyl), P site (peptidyl), E site (exit)
• Initiation → Elongation → Termination
• Wobble hypothesis (Crick): 3rd base of codon can mismatch

• Point mutations: Missense (wrong AA), Nonsense (stop codon), Silent (same AA)
• Frameshift: insertion/deletion → shifts reading frame
• Transition: purine↔purine or pyrimidine↔pyrimidine
• Transversion: purine↔pyrimidine
• Clinical: Sickle cell anemia = missense mutation (Glu→Val, codon 6, β-globin)

• Lac operon (prokaryote model): Repressor model
  • Structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase)
  • When lactose present: inducer binds repressor → repressor leaves operator → genes expressed
• Eukaryotes: Enhancers, silencers, transcription factors, methylation, histones

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🔴 TOPIC 3: Molecular Biology Techniques (13 marks)

Most Important Questions:

• Amplifies specific DNA sequences
• 3 steps (repeated ~30 cycles):
  1. Denaturation (~94°C): strands separate
  2. Annealing (~55°C): primers bind template
  3. Extension (~72°C): Taq polymerase extends
• Result: Exponential amplification (2ⁿ copies)
• Applications: Diagnosis of infections (HIV, TB), genetic diseases, forensics, paternity testing, COVID-19 testing

• General steps: Gel electrophoresis → Transfer to nitrocellulose membrane → Hybridization with probe → Detection

• Restriction endonucleases cut DNA at specific palindromic sequences
• Differences in fragment lengths = genetic polymorphisms
• Used in: Genetic fingerprinting, diagnosis of sickle cell disease, paternity testing

• Uses chain-terminating ddNTPs (dideoxynucleotides)
• Four reactions with ddATP, ddTTP, ddGTP, ddCTP
• Gel electrophoresis separates fragments → sequence read

• Steps: Isolation of gene → Restriction enzyme cuts → Ligation into vector (plasmid) → Transformation into host → Selection → Expression
• Vectors: Plasmids, bacteriophages, cosmids, YACs
• Applications: Insulin production (1st recombinant protein), HGH, erythropoietin, Hepatitis B vaccine

• Gene editing tool: guide RNA directs Cas9 nuclease to specific DNA sequence → cuts DNA
• Used for gene therapy, cancer research

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🔴 TOPIC 4: Heme Metabolism (13 marks)

Most Important Questions:

• Occurs in: Liver (for cytochromes) and Bone marrow (for Hb)
• Steps:
  1. Glycine + Succinyl CoA → δ-ALA (enzyme: ALA synthase - rate-limiting, requires PLP/B6)
  2. 2 ALA → Porphobilinogen (PBG) - ALA dehydratase (inhibited by lead)
  3. 4 PBG → Hydroxymethylbilane → Uroporphyrinogen III → Coproporphyrinogen III → Protoporphyrin IX
  4. Protoporphyrin IX + Fe²⁺ → Heme (enzyme: Ferrochelatase, inhibited by lead)

• Acute Intermittent Porphyria (AIP): PBG deaminase deficiency; abdominal pain, neuropsychiatric symptoms, port-wine urine; No photosensitivity
• Porphyria Cutanea Tarda (PCT): Uroporphyrinogen decarboxylase deficiency; most common porphyria; photosensitivity, blistering skin
• Congenital Erythropoietic Porphyria: Uroporphyrinogen III cosynthase deficiency; red urine, severe photosensitivity, hemolytic anemia

• RBC destruction → Heme → Biliverdin (by Heme oxygenase) → Unconjugated bilirubin (UCB)
• UCB is insoluble, carried in blood bound to albumin
• In liver: UCB + glucuronic acid (UGT enzyme) → Conjugated bilirubin (CB) = direct bilirubin
• CB excreted in bile → intestine → Urobilinogen → Stercobilin (feces, brown color) + Urobilin (urine, yellow)

• Physiological: Days 2-4 due to immature liver UGT
• Hemolytic disease: Rh incompatibility → severe UCB
• Kernicterus: UCB deposits in basal ganglia → neurological damage
• Treatment: Phototherapy (converts UCB to lumirubin, water-soluble)

• Inhibits: ALA dehydratase and Ferrochelatase
• ALA and Coproporphyrin III accumulate in urine
• Features: Basophilic stippling of RBCs, microcytic hypochromic anemia, Burton's lines (gums), encephalopathy
• Treatment: Chelation therapy (DMSA, EDTA)

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🔴 TOPIC 5: Organ Function Tests (13 marks)

Most Important Questions:

• Blood Urea Nitrogen (BUN): 7-20 mg/dL; ↑ in renal failure, dehydration
• Serum Creatinine: 0.6-1.2 mg/dL; best index of GFR; not affected by diet
• GFR: Normal ~125 mL/min; CKD if <60 mL/min for >3 months
• Uric acid: End product of purine catabolism; ↑ in gout
• Creatinine clearance = [U × V] / P (gold standard for GFR estimation)
• Urine: Proteinuria, hematuria, casts in nephritis

• TSH: Most sensitive test (0.4-4.0 mIU/L)
• Free T4 (fT4) and Free T3 (fT3)
• Hypothyroid: ↑TSH, ↓fT4 | Hyperthyroid: ↓TSH, ↑fT4
• T3 is metabolically more active, T4 is converted to T3 in tissues

• Troponin = most specific and sensitive for myocardial damage

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🟠 TOPIC 6: Nucleotide Metabolism (10 marks)

Most Important Questions:

• Purine ring is built atom by atom on PRPP (phosphoribosyl pyrophosphate)
• Key reactions: 10 steps; end product = IMP → AMP or GMP
• Rate-limiting enzyme: PRPP amidotransferase (inhibited by AMP, GMP - feedback)
• Key donors: Glutamine (N), glycine, formate (C10-THF), CO2, aspartate

• Recycles free purine bases → nucleotides
• HGPRT (Hypoxanthine-Guanine Phosphoribosyl Transferase) - converts hypoxanthine → IMP, guanine → GMP
• Lesch-Nyhan syndrome: HGPRT deficiency → hypoxanthine not salvaged → excess uric acid production → gout + neurological features (self-mutilation, choreoathetosis, intellectual disability)

• Purines → Hypoxanthine → Xanthine → Uric Acid (enzyme: Xanthine oxidase)
• Gout: Uric acid crystals deposit in joints (esp. big toe = podagra), tophi, kidney stones
• Treatment: Allopurinol (xanthine oxidase inhibitor); Colchicine (acute attack)

• Ring is first synthesized, then attached to ribose
• CAD enzyme (Carbamoyl phosphate synthetase II, Aspartate transcarbamylase, Dihydroorotase) - rate-limiting complex
• End product: UMP → UDP → UTP → CTP
• Orotic aciduria: Deficiency of UMP synthase → orotic acid in urine, megaloblastic anemia

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🟠 TOPIC 7: Biochemistry of Cancer (10 marks)

Most Important Questions:

• Proto-oncogenes → Oncogenes when mutated (accelerators of cell growth)
  • Examples: ras (most commonly mutated in cancer), myc, her2/neu, bcr-abl
  • bcr-abl: Philadelphia chromosome in CML; target of Imatinib
• Tumor suppressor genes = brakes for cell growth
  • p53 (guardian of the genome): mutated in >50% of cancers; triggers apoptosis at G1
  • Rb (retinoblastoma protein): mutated in retinoblastoma, osteosarcoma

• Cancer cells preferentially use aerobic glycolysis (glucose → lactate) even when O2 is available
• Reason: Need for biosynthetic precursors (pentose phosphate pathway, lipid synthesis)
• PET scan exploits this: ¹⁸FDG (glucose analog) taken up more by cancer cells

• G1 → S → G2 → M phases
• Cyclins and CDKs drive progression
• Checkpoints: G1/S (Rb-E2F), G2/M (p53)
• Apoptosis: Intrinsic (Bcl-2/Bax, mitochondria) and extrinsic (death receptor, FasL) pathways

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🟡 TOPIC 8: Protein Chemistry (6 marks)

Most Important Questions:

• Primary: AA sequence (peptide bonds)
• Secondary: α-helix (H-bonds within same chain, 3.6 AA/turn), β-pleated sheet (H-bonds between adjacent chains)
• Tertiary: 3D folding; stabilized by disulfide bonds, hydrophobic interactions, H-bonds, ionic bonds
• Quaternary: Multiple subunits (e.g., Hb = 2α + 2β)

• Loss of 3D structure without breaking peptide bonds
• Agents: Heat, strong acids/bases, organic solvents, heavy metals, urea
• Renaturation = reversible denaturation

• Missense mutation: codon 6 of β-globin: GAG → GTG (Glu → Val)
• In deoxygenated state: HbS polymerizes → sickle-shaped RBCs
• Features: Chronic hemolytic anemia, vaso-occlusive crises, splenomegaly, stroke
• HbS in sickle cell: Hydrophobic valine creates sticky patch

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🟡 TOPIC 9: Nucleic Acid Chemistry (6 marks)

Most Important Questions:

• Double helix, antiparallel strands
• B-DNA: right-handed, 10 bp/turn, most common physiological form
• Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)
• Backbone: Sugar (deoxyribose) - Phosphate
• Purines: Adenine, Guanine | Pyrimidines: Cytosine, Thymine (DNA), Uracil (RNA)
• Chargaff's rules: A=T, G=C; %A+G = %T+C (50% purines, 50% pyrimidines)

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🟡 TOPIC 10: Free Radicals and Antioxidants (6 marks)

Most Important Questions:

• Reactive Oxygen Species (ROS): Superoxide (O₂•⁻), Hydroxyl (•OH), Peroxyl, H₂O₂
• Generated by: Mitochondrial electron transport, phagocytosis, radiation, smoking
• Damage: Lipid peroxidation (membrane damage), DNA strand breaks, protein oxidation

• Enzymatic:
  • SOD (Superoxide dismutase): O₂•⁻ → H₂O₂ (contains Mn, Cu, Zn)
  • Catalase: H₂O₂ → H₂O + O₂ (in peroxisomes)
  • Glutathione peroxidase: H₂O₂ + GSH → GSSG (requires Se)
• Non-enzymatic: Vit C, Vit E (lipid-soluble, most important), β-carotene, uric acid, albumin

• ROS → atherosclerosis (LDL oxidation), aging, cancer, reperfusion injury, diabetes complications

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🟢 TOPIC 11: Plasma Proteins (5 marks)

Most Important Questions:

• Albumin (most abundant, 60%): Transport (bilirubin, FFA, drugs, Ca²⁺), maintain colloid osmotic pressure, buffer
• Globulins: α1, α2, β, γ
  • α1: α1-antitrypsin (protease inhibitor), orosomucoid
  • α2: Haptoglobin (binds free Hb), ceruloplasmin (copper transport), α2-macroglobulin
  • β: Transferrin (iron transport), fibronectin, complement
  • γ: Immunoglobulins (IgG, IgA, IgM, IgD, IgE)
• Fibrinogen: Coagulation; converted to fibrin by thrombin

• Rise rapidly in inflammation/infection
• Positive: CRP (best marker), fibrinogen, ferritin, haptoglobin, α1-antitrypsin, SAA
• Negative: Albumin, transferrin ↓ (diverted resources to synthesis of positive APP)
• CRP uses: monitor inflammation, infection, assess response to therapy

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🟢 TOPIC 12: Immunology (5 marks)

Most Important Questions:

• Basic unit: 2 heavy chains + 2 light chains (κ or λ) linked by disulfide bonds
• Fab region: antigen binding | Fc region: complement activation, receptor binding | Ig | Special feature | |----|----------------| | IgG | Most abundant; only one crossing placenta; secondary response | | IgM | Pentamer; first produced in primary response; largest | | IgA | Secretory (saliva, tears, breast milk, gut); dimer | | IgE | Allergy, helminth defense; triggers mast cell degranulation | | IgD | B cell surface receptor |

• Classical pathway: Activated by Ag-Ab complex (IgM/IgG)
• Alternate pathway: Activated by bacterial surfaces, LPS
• Lectin pathway: MBL binds mannose on pathogens
• Final: MAC (Membrane Attack Complex) C5b-9 → lyses cells

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🟢 TOPIC 13: Clinical Chemistry (5 marks)

Most Important Questions:

• Fasting blood glucose: 70-100 mg/dL normal
• Impaired fasting: 100-125 mg/dL; Diabetes: ≥126 mg/dL (fasting, confirmed twice)
• 2-hr OGTT: ≥200 mg/dL = diabetes
• HbA1c: Best measure of long-term glycemic control (3 months); ≥6.5% = diabetes
• Glycosylation: Glucose attaches to N-terminal valine of Hb non-enzymatically

• Total cholesterol: <200 mg/dL desirable
• LDL ("bad"): <100 mg/dL optimal; ↑ risk of atherosclerosis
• HDL ("good"): >60 mg/dL protective
• Triglycerides: <150 mg/dL normal
• Friedewald formula: LDL = Total cholesterol - HDL - (TG/5)

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🔵 TOPIC 14: Xenobiotics and Detoxification (3 marks)

Most Important Questions:

• Phase I (Functionalization): Oxidation, reduction, hydrolysis → adds/exposes functional groups (-OH, -NH₂)
  • Enzymes: Cytochrome P450 (CYP) enzymes in ER of liver
  • Products: May be more or less toxic
• Phase II (Conjugation): Adds polar group → makes highly water-soluble for excretion
  • Glucuronidation (UDP-glucuronosyltransferase - most important)
  • Sulfation, acetylation, methylation, glutathione conjugation
• Phase III: Transporters (efflux pumps, P-glycoprotein) export conjugated products

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📝 RAPID REVISION - KEY MNEMONICS

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⭐ HIGHEST-YIELD 1-LINERS FOR TOMORROW

1. Most common porphyria: Porphyria Cutanea Tarda (PCT)
2. Rate-limiting enzyme of heme synthesis: ALA synthase
3. Rate-limiting enzyme of urea cycle: CPS-I (Carbamoyl Phosphate Synthetase-I)
4. Universal methyl donor: SAM (S-Adenosylmethionine)
5. Lesch-Nyhan = HGPRT deficiency → Self-mutilation + gout
6. Sickle cell = Glu → Val (codon 6, β-globin)
7. Most specific cardiac marker: Troponin I/T
8. Most common oncogene mutated in cancer: ras
9. Guardian of the genome: p53
10. Most abundant plasma protein: Albumin
11. Phototherapy treats neonatal jaundice by converting UCB → lumirubin
12. Only Ig crossing placenta: IgG
13. PCR = Taq polymerase (heat-stable, from Thermus aquaticus)
14. Guthrie test = newborn screening for PKU
15. Allopurinol = xanthine oxidase inhibitor → treats gout

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1. Heme metabolism (full pathway + porphyrias + jaundice table)
2. Molecular biology (central dogma + lac operon)
3. Molecular biology techniques (PCR + blotting table)
4. Protein/AA metabolism (urea cycle + PKU + amino acidopathies)
5. Organ function tests (LFTs + cardiac markers)

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🧪 Detoxification & Xenobiotics - Complete Exam Guide

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What are Xenobiotics?

• Includes: drugs, food additives, environmental pollutants, cosmetics, pesticides, plant toxins
• Most are hydrophobic (lipid-soluble) → easily absorbed but hard to excrete
• Goal of detoxification: convert them to hydrophilic (water-soluble) products → excrete in urine or bile

"In general, most xenobiotics are hydrophobic chemicals; in the absence of metabolism, these would not be efficiently eliminated and thus would accumulate in the body, potentially resulting in toxicity." - Goodman & Gilman's

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Primary Site of Detoxification

• Phase I enzymes → Smooth Endoplasmic Reticulum (SER)
• Phase II enzymes → Cytoplasm + microsomes

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THE THREE PHASES OF BIOTRANSFORMATION

Xenobiotic (lipophilic)
        ↓
   PHASE I: Functionalization
        ↓
   Intermediate metabolite (may expose reactive groups)
        ↓
   PHASE II: Conjugation
        ↓
   Polar conjugate (water-soluble)
        ↓
   PHASE III: Transport/Excretion
        ↓
   Excreted in urine or bile

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⚡ PHASE I - Functionalization Reactions

Types of Phase I Reactions:

🔑 CYTOCHROME P450 (CYP) System - THE MOST IMPORTANT

• Location: Smooth ER of hepatocytes
• Superfamily of heme-containing monooxygenases
• Overall reaction:

  Drug (RH) + O₂ + NADPH + H⁺ → Drug-OH (ROH) + H₂O + NADP⁺
  

• CYP3A4 = most important isoform → metabolizes >50% of all prescription drugs
• Other important isoforms:

Phase I - Detailed Oxidation Reactions

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⚡ PHASE II - Conjugation Reactions

Phase II Conjugation Reactions (EXAM TABLE):

Glucuronidation = most important Phase II reaction (handles most drugs including bilirubin)

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⚡ PHASE III - Transport

• P-glycoprotein (P-gp / MDR1) and other ABC transporters in intestine, liver, kidney
• Efflux conjugated products OUT of cells into bile or urine
• Clinically important: P-gp overexpression → multidrug resistance in cancer

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🔬 ENZYME INDUCTION & INHIBITION

Enzyme Inducers (↑ CYP expression → ↓ drug effect)

Enzyme Inhibitors (↓ CYP → ↑ drug toxicity)

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🏥 CLINICALLY IMPORTANT EXAMPLES

1. Paracetamol (Acetaminophen) Toxicity - MUST KNOW!

Paracetamol
    ↓ (90%) Phase II → Glucuronide/Sulfate conjugates (safe, excreted)
    ↓ (10%) Phase I CYP2E1/3A4 → NAPQI (N-acetyl-p-benzoquinone imine)
                                    ↓
                          Normally → Conjugated with GSH (safe)
                          Overdose → GSH depleted → NAPQI accumulates
                                    → Hepatic necrosis (centrilobular)

ANTIDOTE: N-Acetylcysteine (NAC) → replenishes GSH

2. Prodrugs activated by Phase I

3. Isoniazid (INH) Acetylation Polymorphism

• NAT2 gene polymorphism → fast vs. slow acetylators
• Slow acetylators: Higher blood levels → peripheral neuropathy (treat with Vit B6/pyridoxine)
• Fast acetylators: Rapid clearance → may need higher doses; more hepatotoxic metabolites

4. First-Pass Effect

• Drug absorbed from gut enters portal circulation → liver before systemic circulation
• High first-pass drugs: Morphine, propranolol, GTN, lidocaine, aspirin
• Why GTN is given sublingually (bypasses first-pass)

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🧬 FACTORS AFFECTING BIOTRANSFORMATION

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⭐ HIGH-YIELD EXAM POINTS - RAPID REVISION

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📝 LIKELY EXAM QUESTIONS

1. Explain Phase I and Phase II reactions of detoxification with examples
2. What is biotransformation? Describe the role of cytochrome P450
3. Explain enzyme induction with clinical examples
4. Write a note on paracetamol toxicity and its mechanism
5. What are prodrugs? Give examples of bioactivation

1. Describe the phases of biotransformation of xenobiotics. Add a note on factors affecting biotransformation
2. Explain in detail the role of liver in detoxification of foreign compounds

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Xenobiotic (lipophilic, hard to excrete)
        ↓ Phase I (CYP450 in SER) - Oxidation/Reduction/Hydrolysis
        ↓ adds -OH, -NH₂, -COOH groups
        ↓ Phase II (Transferases in cytoplasm) - Conjugation
        ↓ adds glucuronic acid / sulfate / glutathione / acetyl / methyl
        ↓ Phase III (P-gp transporters)
Hydrophilic product → Urine (kidney) or Bile (liver) → Excreted

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🏥 Organ Function Tests - Complete Exam Guide (13 Marks)

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OVERVIEW - What are Function Tests?

• Diagnosis of disease
• Assessing prognosis
• Monitoring treatment

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🔴 1. LIVER FUNCTION TESTS (LFTs)

┌─────────────────────────────────────────────┐
│  Liver Functions Tested:                    │
│  1. Hepatocyte INTEGRITY → Enzymes (ALT,AST)│
│  2. Biliary EXCRETION → Bilirubin, ALP, GGT │
│  3. Synthetic FUNCTION → Albumin, PT        │
└─────────────────────────────────────────────┘

1A. Tests of Hepatocyte Integrity (Damage Markers)

✏️ "ALT is more specific for liver disease than AST because AST is elevated in cardiac or skeletal muscle injury while ALT is not." - Harper's Biochemistry

• De Ritis ratio (AST:ALT)
  • AST:ALT > 2:1 → Alcoholic liver disease
  • AST:ALT < 1 → Viral hepatitis (ALT rises more)

1B. Tests of Biliary Excretion

1C. Tests of Synthetic Function

Key point: In acute liver failure → PT/INR is best marker (clotting factors have short half-life) In chronic liver failure → Albumin + bilirubin + PT together (Child-Pugh score)

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🔴 2. KIDNEY FUNCTION TESTS (RFTs / KFTs)

Key concept: GFR = Glomerular Filtration Rate

• Normal GFR = 125 mL/min (or ~180 L/day filtered, 1.5 L excreted)
• CKD defined as GFR <60 mL/min for >3 months

2A. Blood Tests

"Creatinine is produced at a relatively constant rate by muscle and is freely filtered at the glomerulus... Serum creatinine has an inverse, non-linear relationship with GFR - a large reduction in GFR (e.g., 75%) may only cause a modest rise in serum creatinine" - Barash Clinical Anesthesia

2B. Urine Tests

2C. Clearance Tests (GFR Measurement)

Creatinine Clearance (mL/min) = [Urine creatinine (mg/dL) × Urine volume (mL/min)]
                                 ──────────────────────────────────────────────────
                                         Serum creatinine (mg/dL)

• Normal: 85-125 mL/min
• Overestimates GFR slightly (because tubules also secrete creatinine)

CrCl = [(140 - age) × weight (kg)] / [72 × serum creatinine]
       × 0.85 for females

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🟠 3. THYROID FUNCTION TESTS (TFTs)

Hormones to Know:

• T4 (thyroxine/tetraiodothyronine): Main secretory product; less active
• T3 (triiodothyronine): 3-4x more active; mainly from peripheral conversion of T4
• TSH: From anterior pituitary; stimulates T3/T4 production; most sensitive test

Tests:

✏️ "Total thyroxine is seldom measured nowadays, because assays to measure free thyroxine are available." - Harper's Biochemistry

Interpretation:

• Anti-TPO antibodies → Hashimoto's thyroiditis
• Anti-TSH receptor antibodies → Graves' disease
• Radioactive iodine uptake (RAIU) → ↑ in Graves'; ↓ in thyroiditis

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🟠 4. CARDIAC FUNCTION TESTS

4A. Cardiac Markers (for MI / Myocardial Infarction)

✏️ "An ECG may not always show typical changes following a myocardial infarction. In such a situation, elevation in serum levels of cardiac troponin or creatine kinase MB isoenzyme provides confirmation." - Harper's Biochemistry

4B. Lipid Profile (Cardiovascular risk assessment)

LDL = Total Cholesterol - HDL - (TG/5)
(Only valid when TG < 400 mg/dL)

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🟡 5. PANCREATIC FUNCTION TESTS

✏️ "Lipase measurements are recommended over amylase measurements to diagnose acute pancreatitis because of their greater sensitivity and specificity." - Textbook of Family Medicine

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🟡 6. ADRENAL FUNCTION TESTS

✏️ "Loss of diurnal variation is one of the earliest signs of adrenal hyperfunction" - Harper's Biochemistry Normal: Cortisol highest at 8 AM, lowest at midnight

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⭐ HIGH-YIELD RAPID REVISION TABLE

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📝 LIKELY EXAM QUESTIONS

1. Write briefly on Liver Function Tests
2. Enumerate the tests used for assessing renal function with normal values
3. What is creatinine clearance? How is it calculated?
4. Write a note on cardiac markers in myocardial infarction
5. Describe thyroid function tests

1. Describe in detail the biochemical tests used to assess liver function. How do they help differentiate types of jaundice?
2. Write in detail about organ function tests including liver, kidney, and cardiac markers

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Memory Tricks 🧠