Q3 - Full Answer: Scurvy (Vitamin C Deficiency)

Clinical Case Summary

• Bleeding gums, swollen gums
• Joint pain and fatigue
• Petechiae over the lower limbs
• Splinter hemorrhages under nails

a) Most Probable Diagnosis (1 mark)

b) Biochemical Basis of Symptoms (3 marks)

How Vitamin C works normally:

• Prolyl hydroxylase - hydroxylates proline residues to hydroxyproline
• Lysyl hydroxylase - hydroxylates lysine residues to hydroxylysine

What goes wrong in Vitamin C deficiency:

• Bleeding/swollen gums: Gum tissue is rich in collagen. Defective collagen in the gingival vessels and connective tissue causes capillary fragility and gingival hemorrhage.
• Petechiae and splinter hemorrhages: Blood vessel walls lack adequate collagen support, so minor trauma causes pinpoint bleeds (petechiae) and linear bleeds under nails (splinter hemorrhages). Collagen, having the highest content of hydroxyproline of any protein, is most severely affected in blood vessels.
• Joint pain: Periosteal and periarticular collagen is defective, causing subperiosteal hemorrhages and joint effusions, especially in growing bones.
• Fatigue/anemia: Vitamin C is required for the synthesis of carnitine (via hydroxylation steps) and for norepinephrine synthesis (dopamine beta-hydroxylase). Deficiency impairs energy metabolism. Vitamin C also enhances non-heme iron absorption; its absence contributes to iron-deficiency anemia.
• Robbins & Kumar Basic Pathology, p. 296-297
• Tietz Textbook of Laboratory Medicine, 7th Ed, p. 1303

c) Role of the Deficient Agent (Vitamin C) in Collagen Synthesis (4 marks)

Steps of Collagen Synthesis and Where Vitamin C Acts:

1. Transcription and Translation: The COL genes are transcribed and translated on the RER to produce preprocollagen (with a signal peptide).
2. Hydroxylation of Proline and Lysine (VITAMIN C IS REQUIRED HERE):
   • Inside the ER, prolyl 4-hydroxylase and lysyl hydroxylase act on proline and lysine residues.
   • Vitamin C (ascorbate) keeps the Fe²⁺ ion in the enzyme's active site in its reduced state, preventing oxidation to Fe³⁺ which would inactivate the enzyme.
   • This produces hydroxyproline and hydroxylysine.
   • Hydroxyproline stabilizes the triple helix via hydrogen bonds.
   • Hydroxylysine provides sites for cross-linking and glycosylation.
3. Triple Helix Formation:
   • Three alpha chains associate at their C-terminus and wind into a right-handed triple helix stabilized by hydroxyproline-dependent hydrogen bonds.
   • Without adequate hydroxylation, the helix is unstable and cannot be properly secreted.
4. Secretion and Cleavage:
   • Procollagen is secreted into the extracellular space.
   • Procollagen peptidases cleave the propeptides to yield tropocollagen.
5. Cross-linking (VITAMIN C IS REQUIRED HERE INDIRECTLY):
   • Lysyl oxidase oxidizes hydroxylysine and lysine to form allysine, which then participates in covalent cross-linking between adjacent tropocollagen molecules.
   • Cross-linking gives collagen its tensile strength.
   • Without Vitamin C, inadequate hydroxylysine means poor cross-linking.

• Prolyl and lysyl hydroxylases are inactive
• Procollagen cannot form a stable helix
• Secreted collagen is defective, lacks tensile strength, and is rapidly degraded
• All collagen-rich tissues (blood vessels, skin, bone, gums) become structurally weak
• Robbins & Kumar Basic Pathology, p. 296
• Harper's Illustrated Biochemistry, 32nd Ed
• Tietz Textbook of Laboratory Medicine, 7th Ed, p. 1303

d) Dietary Sources and RDA of Vitamin C in Children (2 marks)

Dietary Sources (rich sources first):

RDA for Vitamin C (as per US Institute of Medicine / ICMR):

• Tietz Textbook of Laboratory Medicine, 7th Ed, p. 1303-1304
• Harper's Illustrated Biochemistry, 32nd Ed

Summary Box

Mechanism of Action of Steroid Hormones

Overview

Step-by-Step Mechanism

Step 1 - Synthesis and Transport

• Steroid hormones are synthesized in endocrine glands (adrenal cortex, gonads, placenta).
• They are lipid-soluble and do not dissolve freely in plasma; they are carried bound to carrier proteins (e.g., cortisol-binding globulin, sex hormone-binding globulin).
• The free (unbound) fraction is biologically active.

Step 2 - Membrane Crossing

• The free hormone diffuses passively across the lipid bilayer of the target cell membrane - no receptor or transporter needed.
• (Thyroid hormones, though not true steroids, use carrier-mediated transporters to enter cells.)

Step 3 - Binding to the Cytoplasmic Receptor (Type I - Glucocorticoid/Mineralocorticoid/Sex Hormone Model)

• Hsp90 keeps the receptor in the cytoplasm and masks the nuclear localization signal (NLS).
• Hsp90 also maintains the ligand-binding domain in a conformation that has high affinity for the hormone.

• Hsp90 dissociates from the receptor.
• The Nuclear Localization Signal (NLS) is exposed.
• A conformational change occurs in the receptor.

Step 4 - Dimerization and Nuclear Translocation

• Two hormone-receptor complexes come together and form a homodimer (for glucocorticoid, androgen, progesterone, estrogen receptors).
• The dimer is transported through the nuclear pore into the nucleus via the exposed NLS.

Step 5 - Binding to Hormone Response Element (HRE)

• Each class of steroid hormone has its own specific HRE sequence.
• HREs are typically palindromic hexanucleotide sequences.

Step 6 - Transcriptional Activation or Repression

1. Coactivators (e.g., SRC/p160 family, CBP/p300) - which have Histone Acetyltransferase (HAT) activity - loosen chromatin structure, making DNA more accessible.
2. Mediator proteins / TRAP220 - which recruit the Basal Transcription Complex (RNA Pol II + general transcription factors) to the TATA box.
3. Chromatin remodeling complexes (e.g., SWI/SNF via Brg1) - slide nucleosomes off DNA.
4. Histone methyltransferases (e.g., CARM1) - further enhance transcription.

• The mRNA produced is then translated into new effector proteins that produce the biological response.

Step 7 (Type II Receptors - Thyroid Hormone / Retinoic Acid Model - Contrast)

• They form heterodimers with Retinoid X Receptor (RXR) and are constitutively bound to HREs.
• In the absence of hormone, they bind corepressors (with HDAC activity) and repress transcription.
• When the hormone binds, the receptor undergoes a conformational change, corepressors are released, coactivators are recruited, and transcription is activated.

Domains of the Steroid Hormone Receptor

Key Features Compared to Peptide Hormone Signaling

Biological Effects (Examples)

• Glucocorticoids (cortisol): Anti-inflammatory genes upregulated; pro-inflammatory genes (IL-2, TNF-α) downregulated via transrepression.
• Estrogen: Upregulates genes for uterine growth, breast development, LH receptor synthesis.
• Testosterone: Promotes muscle protein synthesis, spermatogenesis genes.
• Aldosterone: Upregulates ENaC (epithelial Na⁺ channel) and Na⁺/K⁺-ATPase genes in kidney tubules.
• Vitamin D (1,25-OH₂D₃): Upregulates calcium-binding proteins in the gut for Ca²⁺ absorption.

Summary Flowchart

Steroid hormone (lipophilic)
        ↓
Diffuses across plasma membrane
        ↓
Binds cytoplasmic receptor (displaces Hsp90)
        ↓
NLS exposed → Receptor dimerizes
        ↓
Translocates into nucleus via nuclear pore
        ↓
DBD binds HRE (specific DNA sequence)
        ↓
TAD recruits coactivators + chromatin remodelers
        ↓
RNA Pol II basal complex assembled
        ↓
Gene transcription ↑ or ↓
        ↓
New mRNA → New proteins → Biological effect

• Basic Medical Biochemistry: A Clinical Approach, 6th Ed, p. 517-518
• Medical Physiology (Boron & Boulpaep), p. 143-144

Liver Function Tests (LFTs) - Complete Notes for 1st Year MBBS

What are LFTs?

"The best way to increase sensitivity and specificity is to employ a battery of tests that includes aminotransferases, alkaline phosphatase, bilirubin, albumin, and prothrombin time." - Harrison's Principles of Internal Medicine, 22nd Ed

Classification of LFTs

1. TESTS OF HEPATOCYTE INTEGRITY (Cell Damage Tests)

A. Serum Transaminases (Aminotransferases)

Alanine Aminotransferase (ALT) - also called SGPT

• Location: Predominantly in liver cytoplasm (liver-specific)
• Normal: 7-56 U/L
• Rises in: Acute hepatitis, hepatocellular necrosis, drug toxicity
• Key point: ALT is more specific for liver disease than AST because AST is also found in heart and skeletal muscle

Aspartate Aminotransferase (AST) - also called SGOT

• Location: Liver, heart, skeletal muscle, kidney (less specific)
• Normal: 10-40 U/L
• Rises in: Liver disease, MI, muscle injury
• Key ratio - AST:ALT >2:1 strongly suggests alcoholic liver disease or cirrhosis

ALT > AST = viral hepatitis; AST:ALT > 2 = alcoholic hepatitis/cirrhosis

Lactate Dehydrogenase (LDH)

• Non-specific enzyme; rises in hepatic necrosis, hemolysis, malignancy
• Less commonly used now; elevated in space-occupying lesions of liver

2. TESTS OF BILIARY EXCRETORY FUNCTION

A. Serum Bilirubin

• Normal total bilirubin: 0.2-1.2 mg/dL
• Jaundice appears clinically when bilirubin >2.5-3 mg/dL
• Urine bilirubin: Only conjugated (direct) bilirubin appears in urine (dark urine in obstructive jaundice); absent in hemolytic jaundice

B. Alkaline Phosphatase (ALP)

• Located on the canalicular surface of hepatocytes
• Normal: 44-147 IU/L
• Rises markedly (>4x normal) in: Obstructive jaundice (stones, tumors blocking bile duct), infiltrative liver disease
• Also raised in: Bone disease (Paget's disease, bone metastases) - if uncertain about source, confirm with GGT or 5'-nucleotidase

C. Gamma-Glutamyl Transferase (GGT)

• Located on the hepatic surface of the canaliculus
• Normal: 9-48 U/L
• Very sensitive marker; rises early in:
  • Obstructive jaundice
  • Alcoholic liver disease (most sensitive marker for alcohol use)
  • Drug-induced liver disease
• Used to confirm liver origin of elevated ALP (bone disease does NOT raise GGT)

3. TESTS OF HEPATOCYTE SYNTHETIC FUNCTION

A. Serum Albumin

• Entirely synthesized in the liver
• Normal: 3.5-5.0 g/dL (35-50 g/L)
• Half-life = ~20 days (reflects chronic liver function)
• Low albumin = chronic liver disease (cirrhosis), malnutrition, nephrotic syndrome
• NOT useful in acute liver disease (takes weeks to fall)

B. Prothrombin Time (PT) / INR

• Liver synthesizes coagulation factors II, V, VII, IX, X (all except VIII)
• PT is prolonged when liver cannot make enough clotting factors
• Half-life of Factor VII = ~6 hours → PT reflects acute liver function
• Key clinical point:
  • PT prolonged + corrects with Vitamin K = obstructive jaundice (fat-soluble Vitamin K not being absorbed)
  • PT prolonged + does NOT correct with Vitamin K = hepatocellular damage (liver can't synthesize factors even with adequate Vitamin K)

C. Serum Ammonia

• Liver converts toxic ammonia to urea via the urea cycle (Krebs-Henseleit cycle)
• Rises only when >80% liver tissue is destroyed
• Elevated ammonia → Hepatic Encephalopathy (confusion, altered consciousness, flapping tremor - asterixis)
• Useful in cirrhosis and fulminant hepatic failure

4. DISORDERS ASSOCIATED WITH LFT PATTERNS

• Harrison's Principles of Internal Medicine, 22nd Ed, Table 348-1
• Henry's Clinical Diagnosis and Management by Laboratory Methods

Specific Disorder Discussions

1. Acute Viral Hepatitis

• AST and ALT rise markedly (>500 IU/L), days before jaundice appears
• ALT > AST (liver-specific injury)
• <80% liver destroyed → albumin and ammonia remain normal
• Self-limiting; full regeneration occurs

2. Cirrhosis

• Fibrosis replaces hepatocytes → no regeneration → >80% tissue lost
• AST/ALT are normal or mildly elevated (few cells left to lyse)
• Albumin ↓, Ammonia ↑, PT prolonged = signs of failed synthetic function
• Both conjugated + unconjugated bilirubin elevated

3. Obstructive (Cholestatic) Jaundice

• Caused by gallstones, carcinoma of head of pancreas, bile duct stricture
• ALP and GGT rise dramatically (canalicular enzymes backed up)
• Direct bilirubin rises predominantly (conjugated bilirubin cannot be excreted)
• Dark urine (bilirubinuria), pale stools (no stercobilin)
• PT prolonged but corrects with parenteral Vitamin K (absorption problem, not synthesis)

4. Alcoholic Liver Disease

• Classic finding: AST:ALT ratio >2:1
• GGT markedly elevated (most sensitive marker for alcohol use)
• Macrocytosis on CBC is an additional clue

5. Fulminant Hepatic Failure

• Most severe - total liver failure
• AST/ALT >10,000 IU/L + AST disproportionately > ALT
• Albumin markedly low, Ammonia very high → hepatic encephalopathy
• PT markedly prolonged, doesn't correct with Vit K
• Medical emergency → requires liver transplantation

Memory Aid - Quick Summary

LFT Tests grouped by function:

DAMAGE tests (↑ = bad):
  AST (SGOT) - less specific
  ALT (SGPT) - more specific for liver
  LDH

BILE tests (↑ = bad):
  Bilirubin (direct/indirect/total)
  ALP - obstructive
  GGT - obstructive + alcohol

SYNTHETIC tests (↓ = bad):
  Albumin (chronic failure)
  PT/INR (acute failure)
  Ammonia (>80% destruction)

• High ALT/AST + normal ALP = Hepatocellular disease (hepatitis)
• High ALP + normal ALT/AST = Cholestatic/Obstructive disease
• Low albumin + prolonged PT = Chronic liver failure (cirrhosis)
• Robbins & Kumar Basic Pathology, Table 14.1, p. 596
• Harper's Illustrated Biochemistry, 32nd Ed, p. 586
• Harrison's Principles of Internal Medicine, 22nd Ed

Monoclonal Antibodies - Complete Notes for 1st Year MBBS

What is an Antibody? (Quick Recap)

Definition of Monoclonal Antibodies

"Monoclonal antibodies are identical antibodies produced by a single clone of cells... the hybridoma is immortal and produces a single (monoclonal) antibody." - Medical Microbiology, 9th Ed

• Produced by a single clone of identical cells
• All structurally identical to each other
• All binding to the exact same epitope of an antigen
• Produced in unlimited quantities

Historical Background

• 1975 - Köhler and Milstein (Cambridge, UK) developed the hybridoma technique for producing monoclonal antibodies from B-cell hybridomas.
• They were awarded the Nobel Prize in Physiology or Medicine in 1984 for this discovery.
• This technology completely transformed immunology, diagnostics, and therapeutics.

Production of Monoclonal Antibodies - Hybridoma Technology

Step-by-Step Production

Step 1 - Immunization of the Mouse

• A mouse (or rat) is injected with the desired antigen (e.g., a tumor protein, viral protein, hormone).
• The mouse mounts an immune response - its B lymphocytes are activated and produce antibodies specific for that antigen.
• A booster injection is given 3 days before sacrifice to maximize antibody-producing cells.

Step 2 - Isolation of Spleen/Lymph Node B Cells

• The mouse is sacrificed.
• The spleen (or lymph nodes) is removed and a suspension of B lymphocytes is prepared.
• These cells produce the desired antibody but will die in culture (mortal cells).

Step 3 - Preparation of Myeloma Cells

• A myeloma cell line (cancerous B cell, immortal) is grown in culture.
• These myeloma cells are specially selected to:
  • Not secrete their own antibodies (so they don't contaminate the product)
  • Lack the enzyme HGPRT (hypoxanthine-guanine phosphoribosyltransferase) - this is the key to selection (explained below)

Step 4 - Cell Fusion using Polyethylene Glycol (PEG)

• Spleen B cells + Myeloma cells are mixed and fused using polyethylene glycol (PEG), a chemical fusogen that promotes cell membrane merging.
• The fusion produces a mixture of:
  • Unfused spleen B cells
  • Unfused myeloma cells
  • Hybridomas (fused B cell + myeloma cell) - the ones we want

Step 5 - Selection in HAT Medium

• The mixture is transferred to HAT medium (Hypoxanthine + Aminopterin + Thymidine).
• Aminopterin blocks the main DNA synthesis pathway (de novo pathway) in ALL cells.
• Cells can survive only if they use the salvage pathway (using HGPRT enzyme).

Step 6 - Screening and Cloning

• Surviving hybridomas are separated into individual wells (single-cell dilution / limiting dilution).
• Each well is tested (screened) for production of the desired antibody using ELISA or other assays.
• Positive clones are selected and expanded.

Step 7 - Mass Production

• The selected hybridoma clone is grown in bulk tissue culture or injected into the peritoneal cavity of mice (as ascites fluid) to produce large amounts.
• The monoclonal antibody is then purified for use.

Production Diagrams (from Janeway's Immunobiology & Goodman & Gilman's)

Types of Monoclonal Antibodies (Based on Origin)

The suffix -mab always denotes a monoclonal antibody in drug names.

• Goodman & Gilman's Pharmacological Basis of Therapeutics

Advantages and Disadvantages

Applications of Monoclonal Antibodies

1. Diagnostic Applications

2. Therapeutic Applications

a) Cancer Treatment (most important use today)

b) Autoimmune / Inflammatory Diseases

c) Transplant Rejection Prevention

• Basiliximab, Daclizumab - target IL-2 receptor on T cells → prevent rejection of transplanted organs

d) Infectious Diseases

• Palivizumab - prevents RSV (Respiratory Syncytial Virus) in premature infants
• COVID-19 neutralizing antibodies - e.g., Sotrovimab

e) Cardiovascular Disease

• Abciximab - targets glycoprotein IIb/IIIa on platelets; prevents clot formation during coronary angioplasty

f) Research Tools

• Used in immunohistochemistry, ELISA, Western blots, flow cytometry - standard tools in all medical research labs

Summary - Key Points for Exam

MONOCLONAL ANTIBODIES
│
├── DEFINITION: Identical antibodies from a single B-cell clone
│              targeting ONE epitope
│
├── PRODUCTION (Hybridoma Technology - Kohler & Milstein, 1975):
│   Mouse immunized → Spleen B cells isolated
│   + Myeloma cells (HGPRT⁻, immortal, no Ab)
│   ↓ Cell fusion with PEG
│   → Hybridoma = immortal + antibody-producing
│   ↓ Selection in HAT medium (kills unfused cells)
│   → Screen for desired antibody → Clone → Mass produce
│
├── TYPES: Murine (-omab) → Chimeric (-ximab)
│          → Humanized (-zumab) → Fully human (-umab)
│
└── APPLICATIONS:
    DIAGNOSTIC: Pregnancy tests, ELISA, blood grouping, cancer IHC
    THERAPEUTIC: Cancer (Rituximab, Trastuzumab), Autoimmune (Infliximab),
                 Transplant (Basiliximab), Infection (Palivizumab)

• Medical Microbiology, 9th Ed (Murray)
• Janeway's Immunobiology, 10th Ed
• Goodman & Gilman's Pharmacological Basis of Therapeutics
• Histology: A Text and Atlas (Ross & Pawlina)