---

Q1 Case: 45-year-old male - Megaloblastic Anemia

• Hb 9g%, RBC 1 million/cc, MCV 100fL (macrocytic), hypersegmented neutrophils, megaloblasts in PBF.

---

• Vitamin B12 deficiency: lack of intrinsic factor (pernicious anemia), gastrectomy, strict vegetarian diet, ileal disease
• Folic acid deficiency: poor diet, malabsorption, increased demand (pregnancy), anti-folate drugs (methotrexate, phenytoin)

---

---

• Macrocytic: MCV >100fL - RBCs are larger than normal due to impaired DNA synthesis causing delayed nuclear maturation but normal cytoplasmic (Hb) development
• Normochromic: MCH and MCHC normal - Hb content per cell is adequate; the defect is in cell division, not hemoglobin synthesis

---

---

1. Dietary B12 released from food by gastric acid and pepsin
2. Binds R-protein (haptocorrin) in stomach
3. In duodenum, pancreatic proteases degrade R-protein; B12 transfers to Intrinsic Factor (IF) secreted by gastric parietal cells
4. B12-IF complex travels to terminal ileum where it binds cubilin receptors and is absorbed
5. Released into portal blood bound to transcobalamin II (TC-II) for transport to tissues
6. In cells: converted to methylcobalamin (cofactor for methionine synthase) and adenosylcobalamin (cofactor for methylmalonyl-CoA mutase)

---

---

• Vitamin B12 (Cyanocobalamin/Hydroxocobalamin): 1000 mcg IM daily for 7 days, then weekly for 4 weeks, then monthly for life (if pernicious anemia)
• Oral B12 can be used in dietary deficiency
• Folic acid supplementation if also deficient (5mg/day orally)
• Treat underlying cause

---

• Folic acid will correct the hematological abnormalities (anemia will improve)
• But the neurological damage (subacute combined degeneration) will continue to progress and may worsen - because B12 deficiency persists and demyelination is unrelated to folate
• This is dangerous - masking the anemia delays diagnosis while nerve damage becomes irreversible
• Never give folic acid alone in B12 deficiency

---

---

Q2. Types of Cell Junctions

---

Q3. Resting Membrane Potential (RMP) and Gibbs-Donnan Equilibrium

1. Unequal ion distribution (maintained by Na+/K+ ATPase pump):
   • Intracellular: high K+ (140 mEq/L), low Na+ (14 mEq/L)
   • Extracellular: high Na+ (142 mEq/L), low K+ (4 mEq/L)
2. Selective membrane permeability: At rest, membrane is predominantly permeable to K+ (via leak channels) → K+ diffuses out down concentration gradient → inside becomes negative
3. Nernst equation: For each ion, equilibrium potential = (61/z) × log([ion]o/[ion]i). For K+: Ek = -94 mV; for Na+: ENa = +61 mV
4. Goldman-Hodgkin-Katz equation: RMP = -70 mV is determined by the relative permeabilities of K+, Na+, Cl-
5. Electrogenic Na+/K+ ATPase: Pumps 3 Na+ out / 2 K+ in → net outward positive charge → contributes ~-5 to -10 mV to RMP

• Applies when one side of a membrane has non-diffusible charged particles (e.g., intracellular proteins which are negatively charged at physiological pH)
• The non-diffusible anions attract cations and repel anions on the same side
• At equilibrium: [K+]i × [Cl-]i = [K+]o × [Cl-]o (product of diffusible ions is equal on both sides)
• Result: more total osmotically active particles inside the cell → tendency for water to enter (prevented by Na+/K+ pump)
• The Donnan effect contributes to the negative intracellular potential and explains why [Cl-] is lower inside the cell
• In the body, the Na+/K+ pump continuously counteracts Donnan equilibrium effects to maintain cell volume

---

Q1. MEGALOBLASTIC ANEMIA - Complete Answers

---

Q.1 Diagnosis [1 mark]

• Hb = 9g% (low), RBC = 1 million/cc (very low)
• MCV = 100fL (macrocytic), MCH = 50pg (high)
• PBF = macrocytic normochromic RBCs + megaloblasts + hypersegmented neutrophils
• Neurological signs = peripheral neuropathy (tingling in hands and feet)
• Angular stomatitis, sore tongue = B12 deficiency signs

---

Q.2 Causes of Megaloblastic Anemia [1 mark]

• Pernicious anemia (lack of Intrinsic Factor)
• Gastrectomy / gastric atrophy
• Strict vegetarian diet
• Terminal ileal disease (Crohn's disease)
• Fish tapeworm (Diphyllobothrium latum)

• Poor dietary intake (commonest cause)
• Malabsorption (celiac disease)
• Increased demand (pregnancy, hemolytic anemia)
• Drugs: methotrexate, phenytoin, trimethoprim

---

Q.3 Cause of Peripheral Neuropathy [1 mark]

• Vitamin B12 (as adenosylcobalamin) is a cofactor for methylmalonyl-CoA mutase
• Deficiency → methylmalonyl-CoA accumulates → abnormal fatty acids incorporated into myelin sheath
• Also: deficiency of methionine (needed for myelin maintenance) due to impaired methionine synthase
• Result: Subacute Combined Degeneration (SACD) of spinal cord
• Clinically: tingling, numbness, peripheral neuropathy, ataxia

Note: Folic acid deficiency does NOT cause neuropathy - this finding points specifically to B12 deficiency.

---

Q.4 Macrocytic and Normochromic RBCs [1 mark]

• MCV > 100 fL (normal 80-100 fL)
• RBCs are larger than normal
• Due to defective DNA synthesis → nuclear maturation is delayed → cell keeps growing without dividing → larger cell

• MCHC is normal
• Hemoglobin content per cell is adequate
• Because the defect is in DNA synthesis only, not in hemoglobin synthesis
• Cytoplasm matures normally while nucleus lags behind (nuclear-cytoplasmic asynchrony)

---

Q.5 Why Megaloblasts (Large Nucleated RBCs) are Seen in PBF [1 mark]

1. Vitamin B12 and Folic acid are required for synthesis of thymidine (via thymidylate synthase)
2. Thymidine is essential for DNA synthesis
3. Deficiency → impaired DNA replication → nucleus cannot divide and mature
4. But RNA and protein (Hb) synthesis continues normally → cytoplasm matures
5. Result: Large immature nucleus in a mature hemoglobin-filled cytoplasm
6. These abnormal precursors (megaloblasts) are released from bone marrow into blood
7. Also: ineffective erythropoiesis → intramedullary destruction of megaloblasts

---

Q.6 Absorption and Utilization of Vitamin B12 [1 mark]

• Methylcobalamin → cofactor for methionine synthase → converts homocysteine to methionine → regenerates THF (active folate for DNA synthesis)
• Adenosylcobalamin → cofactor for methylmalonyl-CoA mutase → converts methylmalonyl-CoA to succinyl-CoA (needed for myelin synthesis and Krebs cycle)

---

Q.7 Why Megaloblastic Anemia Occurs in Gastric Atrophy [1 mark]

1. Gastric atrophy (autoimmune) destroys gastric parietal cells
2. Parietal cells produce Intrinsic Factor (IF)
3. Without IF, B12 cannot form the B12-IF complex
4. B12-IF complex is the only form absorbed in terminal ileum via cubilin receptors
5. Free B12 (without IF) cannot be absorbed → B12 deficiency → megaloblastic anemia

• Autoimmune antibodies against parietal cells and/or IF
• Associated with achlorhydria (no HCl)
• Treated with parenteral (IM) B12, not oral

---

Q.8 Management [1 mark]

• Vitamin B12 (Cyanocobalamin/Hydroxocobalamin):
  • 1000 mcg IM daily × 7 days
  • Then weekly × 4 weeks
  • Then monthly for life (in pernicious anemia)
• Folic acid: 5mg/day orally if also deficient

• Pernicious anemia → lifelong B12 injections
• Dietary deficiency → dietary advice + oral B12

• Blood transfusion only if severe symptomatic anemia
• Monitor reticulocyte count (rises in 3-5 days = good response)
• Monitor neurological recovery

---

Q.9 Effect of Giving Only Folic Acid in Pernicious Anemia [1 mark]

This is why folic acid must never be given alone without ruling out B12 deficiency.

---

Q.10 Mode of Action of Vitamin B12 and Folic Acid in Erythropoiesis [1 mark]

Dietary Folate (polyglutamate)
         ↓ (intestinal conjugase)
Folic acid (monoglutamate)
         ↓ (dihydrofolate reductase - DHFR)
Dihydrofolate (DHF)
         ↓ (DHFR)
Tetrahydrofolate (THF) ← Active form
         ↓
5,10-methylene THF
         ↓ (thymidylate synthase)
dTMP (thymidine) → DNA synthesis ✓

5-methyl THF (methyl trap)
         ↓ + B12 (methionine synthase)
THF regenerated + Methionine formed

• Folic acid (as THF): Carries one-carbon units → donates methyl group for conversion of dUMP → dTMP → DNA synthesis
• Vitamin B12 (as methylcobalamin): Converts 5-methyl THF → THF (regenerates active folate) + homocysteine → methionine
• Both together: Maintain adequate thymidine for DNA replication → normal nuclear division → normal RBC maturation in bone marrow
• Deficiency of either: DNA synthesis fails → nuclear maturation delayed → megaloblastic changes

---

---

Q2. CELL JUNCTIONS [10 marks]

Definition

Classification and Types

1. Tight Junctions (Zonula Occludens)

• Transmembrane proteins: claudins and occludins
• These proteins from adjacent cells fuse together, obliterating the intercellular space
• Appears as a series of fusion points ("kissing points") on electron microscopy

• Acts as a barrier - prevents paracellular movement of molecules between cells
• Separates apical and basolateral membrane domains (fence function)
• Maintains polarity of epithelial cells
• Example: tight junctions of intestinal epithelium prevent luminal contents from leaking into blood

---

2. Adherens Junctions (Zonula Adherens)

• Transmembrane proteins: E-cadherins (Ca²⁺-dependent)
• Intracellular: linked to actin filaments via catenins (α, β, γ catenin)

• Cell-to-cell adhesion
• Maintains epithelial sheet integrity
• Provides mechanical strength against tensile forces
• Important in embryonic development and tissue morphogenesis

---

3. Desmosomes (Macula Adherens)

• Transmembrane proteins: Desmogleins and Desmocollins (cadherins family, Ca²⁺-dependent)
• Intracellular: linked to intermediate filaments (keratin in epithelium, desmin in cardiac muscle) via desmoplakin and plakoglobin
• Dense cytoplasmic plaques on inner leaflet

• Provides strong mechanical attachment - resists shearing and stretching forces
• Especially prominent in skin (epidermis), cardiac muscle, uterine epithelium
• Autoantibodies against desmogleins → Pemphigus vulgaris (blistering skin disease)

---

4. Gap Junctions (Nexus / Communicating Junctions)

• Made of connexins (6 connexin subunits form one connexon/hemichannel)
• Two connexons from adjacent cells align to form a complete channel (1.5-2 nm diameter)
• Allows direct cytoplasmic continuity between cells

• Electrical coupling: Allows ionic current to pass directly → synchronized contraction in cardiac and smooth muscle (functional syncytium)
• Metabolic coupling: Allows passage of small molecules (< 1000 Da) - glucose, amino acids, cAMP, IP3
• Important in: cardiac conduction, uterine contraction during labor, lens nutrition, embryonic development

---

5. Hemidesmosomes

• Transmembrane proteins: integrins (α6β4)
• Linked to intermediate filaments (keratin) intracellularly
• Connects to laminin in the basement membrane extracellularly

• Anchors epithelial cells to the basement membrane (extracellular matrix)
• Autoantibodies → Bullous pemphigoid (subepidermal blistering)

---

Summary Table

Diagram

---

---

Q3. RESTING MEMBRANE POTENTIAL (RMP) AND GIBBS-DONNAN EQUILIBRIUM [10 marks]

Definition of RMP

• Neuron: -70 mV
• Skeletal muscle: -90 mV
• Cardiac muscle: -90 mV
• Smooth muscle: -55 mV

---

Ionic Basis - Concentration Gradients

---

Genesis of RMP

Step 1: Na⁺/K⁺ ATPase Pump

• Pumps 3 Na⁺ out and 2 K⁺ in per cycle using ATP
• Creates and maintains concentration gradients
• Also electrogenic (net outward positive charge) → contributes -5 to -10 mV directly

Step 2: Selective Membrane Permeability at Rest

• At rest, membrane is 25-30 times more permeable to K⁺ than Na⁺ (K⁺ leak channels / inward rectifier K⁺ channels are open)
• K⁺ diffuses out along its concentration gradient
• Each K⁺ leaving makes inside more negative
• Negative inside attracts K⁺ back (electrical force)
• Equilibrium reached when chemical force = electrical force → Nernst equilibrium potential

Step 3: Nernst Equation

E_K = (61/z) × log [K⁺]o / [K⁺]i = (61/1) × log 4/140 = -94 mV

E_Na = 61 × log 142/14 = +61 mV

Step 4: Goldman-Hodgkin-Katz (GHK) Equation

RMP = -70 mV (dominated by K⁺ permeability, pulled slightly positive by Na⁺ and Cl⁻ permeability)

---

Role of Intracellular Proteins

• Large anionic proteins (proteinate⁻) are trapped inside the cell
• They cannot cross the membrane
• They attract cations (K⁺) and contribute to the negative intracellular charge
• This leads into the Gibbs-Donnan effect

---

Gibbs-Donnan Membrane Equilibrium

Definition

Conditions Required

1. A semipermeable membrane
2. Non-diffusible ions on one side (e.g., intracellular proteins, negatively charged)
3. Diffusible ions on both sides (K⁺, Na⁺, Cl⁻)

Donnan Rule

[K⁺]i × [Cl⁻]i = [K⁺]o × [Cl⁻]o

[K⁺]i / [K⁺]o = [Cl⁻]o / [Cl⁻]i

Effects of Donnan Equilibrium

Donnan Equilibrium and Cell Volume

• Donnan effect causes osmotic water entry → tendency for cell to swell indefinitely
• Prevented by Na⁺/K⁺ ATPase pump (pump leak model):
  • Pump keeps Na⁺ low inside → reduces osmotic gradient
  • Without the pump: cell would swell and lyse (as seen when pump is inhibited by ouabain)

Physiological Significance of Donnan Equilibrium

1. Contributes to RMP: Non-diffusible protein anions inside maintain excess negative charge intracellularly
2. Explains Cl⁻ distribution: Cl⁻ is lower inside (Donnan equilibrium keeps it out)
3. Plasma oncotic pressure: In capillaries, plasma proteins (non-diffusible) cause Donnan effect → more ions stay in plasma → contributes to oncotic pressure (Starling forces)
4. Cell volume regulation: Na⁺/K⁺ pump continuously counteracts Donnan-driven swelling

---

Summary of RMP Generation

Na⁺/K⁺ ATPase pump
        ↓
High K⁺ inside, High Na⁺ outside
        ↓
K⁺ leaks out (high permeability at rest)
        ↓
Inside becomes electrically negative
        ↓
Donnan effect of intracellular proteins adds to negativity
        ↓
Electrogenic pump adds -5 to -10 mV
        ↓
RMP = -70 mV (inside negative)

---

Q1. MEGALOBLASTIC ANEMIA

---

Q.1 Diagnosis [1 mark]

• Hb = 9g%, RBC = 1 million/cc → anemia
• MCV = 100fL → macrocytic
• MCH = 50pg → high
• PBF = megaloblasts + hypersegmented neutrophils + decreased platelets
• Peripheral neuropathy + angular stomatitis + sore tongue → B12 deficiency signs

---

Q.2 Causes of Megaloblastic Anemia [1 mark]

• Pernicious anemia (no Intrinsic Factor)
• Gastrectomy / gastric atrophy
• Strict vegetarian diet
• Terminal ileal disease (Crohn's)

• Poor dietary intake
• Malabsorption (celiac disease)
• Increased demand (pregnancy)
• Drugs: methotrexate, phenytoin

---

Q.3 Cause of Peripheral Neuropathy [1 mark]

• B12 (as adenosylcobalamin) is needed for methylmalonyl-CoA mutase
• Deficiency → methylmalonyl-CoA accumulates → abnormal fatty acids incorporated into myelin
• Also: reduced methionine → impaired myelin maintenance
• Result: tingling, numbness, peripheral neuropathy

Folic acid deficiency does NOT cause neuropathy - confirms B12 deficiency

---

Q.4 Macrocytic and Normochromic RBCs [1 mark]

• Defective DNA synthesis → nuclear maturation delayed
• Cell keeps growing without dividing → larger than normal RBC

• Hemoglobin synthesis is normal (iron pathway unaffected)
• Only DNA synthesis is defective
• Cytoplasm matures normally → normal Hb content
• This is called nuclear-cytoplasmic asynchrony

---

Q.5 Why Megaloblasts Seen in PBF [1 mark]

1. B12 + Folic acid required for thymidine synthesis (for DNA)
2. Deficiency → DNA replication fails → nucleus cannot mature/divide
3. Cytoplasm (Hb) matures normally
4. Result: large immature nucleus in mature cytoplasm = Megaloblast
5. Released prematurely from bone marrow into blood
6. Also: ineffective erythropoiesis → intramedullary destruction

---

Q.6 Absorption and Utilization of Vitamin B12 [1 mark]

• Methylcobalamin → methionine synthase → homocysteine → methionine + regenerates THF (active folate)
• Adenosylcobalamin → methylmalonyl-CoA mutase → succinyl-CoA (Krebs cycle + myelin)

---

Q.7 Why This Anemia Occurs in Gastric Atrophy [1 mark]

1. Gastric atrophy (autoimmune) → parietal cells destroyed
2. Parietal cells produce Intrinsic Factor (IF)
3. Without IF → B12 cannot form B12-IF complex
4. B12-IF complex is the only form absorbed in terminal ileum (via cubilin receptors)
5. Free B12 cannot be absorbed → B12 deficiency → Megaloblastic anemia

• Autoantibodies against parietal cells and/or IF
• Associated with achlorhydria
• Treated with IM B12 (not oral)

---

Q.8 Management [1 mark]

• Vitamin B12 IM (Cyanocobalamin 1000 mcg):
  • Daily × 7 days → Weekly × 4 weeks → Monthly for life
• Folic acid 5mg/day orally if deficient

• Pernicious anemia → lifelong IM B12
• Dietary deficiency → oral B12 + diet counseling

• Reticulocyte count rises in 3-5 days (confirms response)
• Neurological recovery (may take months)

---

Q.9 Effect of Only Folic Acid in Pernicious Anemia [1 mark]

Never give folic acid alone without ruling out B12 deficiency

---

Q.10 Mode of Action of B12 and Folic Acid in Erythropoiesis [1 mark]

Dietary Folate
     ↓ DHFR
Tetrahydrofolate (THF) ← Active form
     ↓
5,10-methylene THF
     ↓ Thymidylate synthase
dTMP (Thymidine) → DNA synthesis → Normal RBC division ✓

5-methyl THF (inactive - methyl trap)
     ↓ + Vitamin B12 (methionine synthase)
THF regenerated ✓ + Methionine formed ✓

• Folic acid: Provides one-carbon units → thymidine for DNA synthesis
• Vitamin B12: Regenerates active THF from methyl-trap form
• Both together: Normal DNA synthesis → normal nuclear division → normal RBC maturation
• Deficiency of either: DNA synthesis fails → megaloblastic changes

---

---

Q2. CELL JUNCTIONS [10 marks]

Definition

---

Types of Cell Junctions

1. Tight Junctions (Zonula Occludens)

• Proteins: Claudins and Occludins
• Adjacent cell membranes fuse at multiple points ("kissing points")
• No intercellular space

• Prevents paracellular leakage of molecules
• Maintains epithelial polarity (fence function)
• Example: intestinal epithelium, blood-brain barrier

---

2. Adherens Junctions (Zonula Adherens)

• Proteins: E-Cadherins (Ca²⁺ dependent)
• Linked to actin filaments via catenins intracellularly

• Cell-to-cell adhesion
• Maintains epithelial sheet integrity
• Resists tensile forces

---

3. Desmosomes (Macula Adherens)

• Proteins: Desmogleins + Desmocollins (cadherins, Ca²⁺ dependent)
• Linked to intermediate filaments (keratin/desmin) via desmoplakin
• Dense cytoplasmic plaques visible on EM

• Strong mechanical attachment
• Resists shearing and stretching
• Abundant in skin, cardiac muscle, uterus
• Autoantibodies against desmoglein → Pemphigus vulgaris

---

4. Gap Junctions (Nexus)

• Made of Connexins (6 connexins = 1 connexon/hemichannel)
• Two connexons from adjacent cells align → complete channel (1.5-2 nm diameter)
• Direct cytoplasmic continuity between cells

• Electrical coupling: Ionic current passes directly → synchronized contraction (cardiac syncytium)
• Metabolic coupling: Small molecules (<1000 Da) pass - glucose, cAMP, IP3
• Important in: cardiac conduction, uterine labor contractions, lens nutrition

---

5. Hemidesmosomes

• Proteins: Integrins (α6β4)
• Linked to keratin (intermediate filaments) intracellularly
• Connects to laminin in basement membrane extracellularly

• Anchors epithelial cells to basement membrane
• Autoantibodies → Bullous pemphigoid

---

Summary Table

Diagram

LUMEN
─────────────────────────────────
[Cell 1]    TJ  ←── Tight Junction (Zonula Occludens)
            AJ  ←── Adherens Junction (Zonula Adherens)
            DS  ←── Desmosome (Macula Adherens)
            GJ  ←── Gap Junction
[Cell 1]
─────────────────────────────────
            HD  ←── Hemidesmosome
════════════════════════════════ Basement Membrane

---

---

Q3. RESTING MEMBRANE POTENTIAL AND GIBBS-DONNAN EQUILIBRIUM [10 marks]

Definition of RMP

• Neuron: -70 mV
• Skeletal muscle: -90 mV
• Cardiac muscle: -90 mV
• Smooth muscle: -55 mV

---

Ionic Concentration Gradients

---

Genesis of RMP

1. Na⁺/K⁺ ATPase Pump (Primary)

• Pumps 3 Na⁺ out / 2 K⁺ in per cycle (uses ATP)
• Establishes and maintains concentration gradients
• Net outward positive charge → electrogenic contribution of -5 to -10 mV

2. Selective Membrane Permeability at Rest

• At rest: membrane is 25-30× more permeable to K⁺ than Na⁺ (K⁺ leak channels open)
• K⁺ diffuses outward along concentration gradient
• Each K⁺ leaving → inside becomes more negative
• Negative inside attracts K⁺ back (electrical force)
• Equilibrium: chemical force = electrical force

3. Nernst Equation

E = (61/z) × log [ion]outside / [ion]inside

• For K⁺: E_K = 61 × log (4/140) = -94 mV
• For Na⁺: E_Na = 61 × log (142/14) = +61 mV

4. Goldman-Hodgkin-Katz (GHK) Equation

RMP = -70 mV (dominated by K⁺, slightly offset by Na⁺ and Cl⁻)

5. Intracellular Proteins

• Large anionic proteins trapped inside
• Cannot cross membrane
• Contribute directly to intracellular negativity → Donnan effect

---

Gibbs-Donnan Membrane Equilibrium

Definition

Prerequisites

1. Semipermeable membrane
2. Non-diffusible ions on one side (intracellular proteins, negatively charged)
3. Diffusible ions on both sides (K⁺, Cl⁻, Na⁺)

Donnan Rule

[K⁺]i × [Cl⁻]i = [K⁺]o × [Cl⁻]o

[K⁺]i / [K⁺]o = [Cl⁻]o / [Cl⁻]i

Effects of Donnan Equilibrium

Cell Volume and Donnan Effect (Pump-Leak Model)

• Donnan equilibrium alone would cause endless water entry → cell lysis
• Prevented by Na⁺/K⁺ ATPase pump:
  • Continuously pumps Na⁺ out → keeps intracellular Na⁺ low → reduces osmotic gradient
  • Without pump (e.g., ouabain poisoning) → cell swells and bursts

Physiological Significance

1. Contributes to RMP - intracellular protein anions maintain inside negativity
2. Explains Cl⁻ distribution - Cl⁻ low inside due to Donnan repulsion
3. Cell volume regulation - pump-leak model counters Donnan swelling
4. Plasma oncotic pressure - plasma proteins cause Donnan effect in capillaries → more ions in plasma → contributes to oncotic pressure (Starling's forces)

---

Summary Diagram of RMP Generation

Na⁺/K⁺ ATPase Pump
         ↓
3Na⁺ out / 2K⁺ in
         ↓
High K⁺ inside       High Na⁺ outside
         ↓
K⁺ leaks out via K⁺ leak channels
         ↓
Inside becomes negative
         ↓
Donnan effect of intracellular proteins adds to negativity
         ↓
Electrogenic pump contributes -5 to -10 mV
         ↓
RMP = -70 mV (inside negative)

---

Q1 to Q10 - Simple Point-wise Answers

---

Q.1 What is the patient suffering from?

• Patient is suffering from Megaloblastic Anemia
• Specifically due to Vitamin B12 deficiency
• Also called Pernicious Anemia

---

Q.2 Causes of this type of anemia?

• Lack of Intrinsic Factor (pernicious anemia)
• Gastric atrophy / gastrectomy
• Strict vegetarian diet
• Terminal ileal disease

• Poor diet
• Malabsorption
• Pregnancy (increased demand)
• Drugs - methotrexate, phenytoin

---

Q.3 Cause of peripheral neuropathy?

• B12 deficiency causes demyelination of spinal cord
• Specifically posterior and lateral columns are affected
• This is called Subacute Combined Degeneration (SACD)
• B12 is needed for myelin synthesis
• Without B12 → abnormal fatty acids → myelin breaks down
• Result → tingling, numbness in hands and feet

---

Q.4 What is macrocytic and normochromic RBC?

• RBCs are bigger than normal
• DNA synthesis is defective → cell cannot divide
• Cell keeps growing → becomes large

• Hemoglobin content is normal
• Iron pathway is not affected
• Only DNA is defective, not Hb synthesis
• This is called nuclear-cytoplasmic asynchrony

---

Q.5 Why are megaloblasts seen in PBF?

• B12 and Folic acid are needed for DNA synthesis
• Without them → nucleus cannot mature or divide
• But cytoplasm (Hb) matures normally
• So nucleus stays large and immature → cytoplasm is normal
• This cell = Megaloblast
• These are released from bone marrow into blood
• This is called Nuclear-Cytoplasmic Asynchrony

---

Q.6 How is Vitamin B12 absorbed and utilized?

• Step 1: B12 released from food by HCl and pepsin in stomach
• Step 2: B12 binds R-protein in stomach
• Step 3: In duodenum, R-protein is digested → B12 binds Intrinsic Factor (IF)
• Step 4: B12-IF complex goes to terminal ileum
• Step 5: Binds cubilin receptors → absorbed into blood
• Step 6: Transported by Transcobalamin II to liver and bone marrow

• As methylcobalamin → helps in DNA synthesis (regenerates active folate)
• As adenosylcobalamin → helps in myelin synthesis (methylmalonyl-CoA pathway)

---

Q.7 Why is this anemia seen in gastric atrophy?

• Gastric atrophy destroys parietal cells
• Parietal cells make Intrinsic Factor (IF)
• No IF → B12 cannot be absorbed in terminal ileum
• B12 deficiency → megaloblastic anemia
• This condition is called Pernicious Anemia
• Treated with IM injections of B12 (not oral, because IF is absent)

---

Q.8 How will you manage this patient?

• Vitamin B12 injection (1000 mcg IM)
  • Daily × 7 days
  • Weekly × 4 weeks
  • Monthly for life (if pernicious anemia)
• Folic acid 5 mg/day orally if also deficient

• Treat the underlying cause
• Blood transfusion only if anemia is very severe
• Monitor reticulocyte count (rises in 3-5 days if treatment is working)

---

Q.9 What happens if only folic acid is given in pernicious anemia?

• Anemia will improve - because folate helps DNA synthesis
• But neuropathy will continue to worsen - because B12 is still deficient
• Demyelination of spinal cord progresses
• Blood picture becomes normal → B12 deficiency is masked
• Patient feels better but nerve damage becomes irreversible
• This is dangerous
• So folic acid should never be given alone without ruling out B12 deficiency

---

Q.10 Mode of action of B12 and Folic acid in erythropoiesis?

• Carries one-carbon units
• Needed for synthesis of thymidine (dTMP)
• Thymidine is needed for DNA synthesis
• Without folate → no DNA → no cell division → megaloblasts

• Converts 5-methyl THF → active THF
• Without B12 → folate gets trapped as 5-methyl THF → cannot be used
• This is called Methyl Folate Trap
• So B12 deficiency also causes folate deficiency indirectly

• Both B12 and folic acid → DNA synthesis → normal RBC maturation
• Deficiency of either → impaired DNA → megaloblastic anemia