Physiology Paper 1 - Section B: Complete Answers

Q1 Case: 45-year-old male with indigestion, diarrhea, angular stomatitis, glossitis, peripheral neuropathy

Q.1 - What is this patient suffering from? [1]

Q.2 - What are the causes of this type of anemia? [1]

1. Pernicious anemia - autoimmune gastric atrophy with loss of intrinsic factor (IF) secretion by parietal cells (most common)
2. Gastrectomy - surgical removal eliminates IF-secreting parietal cells
3. Dietary deficiency - strict vegans/vegetarians (B12 is found only in animal products)
4. Ileal disease or resection - ileum is the site of IF-B12 complex absorption
5. Pancreatic insufficiency - pancreatic proteases needed to free B12 from haptocorrin
6. Tapeworm infestation (Diphyllobothrium latum) - competes for B12
7. Drugs - metformin, proton pump inhibitors (reduce absorption)

Guyton & Hall, p. 452; Katzung's Pharmacology 16th ed.

Q.3 - What is the cause of peripheral neuropathy in this type of anemia? [1]

• Posterior and lateral columns of spinal cord
• Peripheral nerves (tingling/numbness in hands and feet as in this patient)

Robbins, Cotran & Kumar Pathologic Basis of Disease; Adams & Victor's Neurology

Q.4 - What do you understand by macrocytic and normochromic RBCs? [1]

• Macrocytic = RBCs larger than normal. Normal MCV is 80-100 fL. This patient has MCV = 100 fL (at the upper limit/macrocytic). The cells are large because B12/folate deficiency impairs DNA synthesis and cell division, but cytoplasmic (Hb) synthesis continues - so cells grow large without dividing. This is called "nuclear-cytoplasmic dissociation."
• Normochromic = normal Hb content per cell. Serum iron is 170 µg/dL (normal/elevated), and MCH = 50 pg (actually elevated, hence technically hyperchromic). The cells carry normal or excess hemoglobin - iron metabolism is intact. There is no iron deficiency.

Guyton & Hall, p. 452

Q.5 - Why do you see large nucleated RBCs (megaloblasts) in PBF in this condition? [1]

• DNA synthesis is impaired (B12 is required for formation of thymidine triphosphate, a DNA building block)
• Nuclear maturation is arrested - the nucleus cannot replicate properly, so the cell cannot divide
• However, RNA and cytoplasmic (Hb) synthesis continue normally - the cell keeps growing
• This results in large immature RBCs with a large, primitive nucleus = megaloblasts
• These are normally only found in bone marrow; their presence in peripheral blood (PBF) indicates severe maturation failure with premature release

Guyton & Hall, p. 452; Brenner & Rector's The Kidney

Q.6 - How is the deficient nutrient (Vitamin B12) absorbed and utilized in our body? [1]

1. Dietary B12 (in animal foods) is released from food proteins by gastric acid and salivary binding proteins
2. Free B12 binds to Intrinsic Factor (IF) - a glycoprotein secreted by parietal cells of gastric glands
3. The B12-IF complex is resistant to proteolytic digestion and travels to the terminal ileum
4. In the ileum, the B12-IF complex binds to specific cubilin receptors on the brush border of ileal mucosal cells
5. B12 is absorbed into blood by pinocytosis and transported bound to transcobalamin II
6. Stored in large quantities in the liver (enough for 3-4 years)

• Co-factor for methionine synthase: converts homocysteine → methionine and regenerates tetrahydrofolate (THF) for DNA synthesis
• Co-factor for methylmalonyl-CoA mutase: converts methylmalonyl-CoA → succinyl-CoA (required for myelin synthesis and fatty acid metabolism)
• Essential for erythropoiesis: via DNA synthesis (thymidine triphosphate formation)

Guyton & Hall, p. 452-453; Costanzo Physiology 7th ed.

Q.7 - Why is this type of anemia seen in patients with gastric atrophy? [1]

• Destruction/atrophy of parietal cells in the gastric mucosa
• Loss of Intrinsic Factor (IF) secretion (parietal cells are the only source of IF)
• Also loss of gastric acid (achlorhydria) - acid is needed to free B12 from food proteins
• Without IF, the B12-IF complex cannot form
• Without this complex, B12 cannot bind to ileal receptors and cannot be absorbed
• Result: B12 deficiency despite adequate dietary intake → megaloblastic anemia

Katzung's Pharmacology 16th ed.; Guyton & Hall p. 452

Q.8 - How will you manage this patient? [1]

1. Vitamin B12 replacement - the mainstay:
   • Intramuscular (IM) cyanocobalamin or hydroxocobalamin injections (bypasses the absorption defect) - 1000 µg daily for 1 week, then weekly for 1 month, then monthly for life (in pernicious anemia)
   • Oral high-dose B12 (1000-2000 µg/day) can be used if cause is dietary deficiency
2. Treat the underlying cause (if addressable)
3. Folic acid supplementation may be given alongside, but NEVER alone (see Q.9)
4. Dietary advice - increase animal products (meat, dairy, eggs)
5. Monitor: reticulocyte count rises in 3-5 days, Hb normalizes in 1-2 months
6. Neurological deficits may partially reverse with early treatment

Q.9 - What will happen if you give only folic acid supplementation in pernicious anemia? [1]

• Folic acid will correct the hematological abnormality (megaloblastic anemia will improve, Hb rises)
• This masks the underlying B12 deficiency
• However, folic acid has no effect on the neurological damage caused by B12 deficiency
• The subacute combined degeneration of the spinal cord and peripheral neuropathy will continue to progress - even worsen - undetected because the blood picture appears to normalize
• The patient may present later with severe, irreversible neurological damage

Lippincott Pharmacology; Robbins & Kumar Basic Pathology

Q.10 - What is the mode of action of Vitamin B12 and Folic Acid in erythropoiesis? [1]

• Acts as cofactor for methionine synthase: converts methyltetrahydrofolate (methyl-THF) → THF, releasing active folate for DNA synthesis
• Also essential for thymidine triphosphate synthesis (via folate metabolism)
• Without B12, folate is "trapped" as methyl-THF ("folate trap") and cannot be used for DNA synthesis

• Converted to tetrahydrofolate (THF) - the active form
• THF donates single-carbon units required for:
  • Synthesis of thymidine monophosphate (TMP) → thymidine triphosphate → DNA
  • Synthesis of purines (adenine and guanine)
• Without folate, pyrimidine and purine synthesis fail → no DNA replication → maturation arrest → megaloblasts

Guyton & Hall p. 452; Brenner & Rector's Kidney - Erythropoiesis section

Q2 - Types of Cell Junctions [10]

1. Occluding Junctions (Tight Junctions / Zonula Occludens)

• Location: Apical region of epithelial cells, forming a belt around the cell
• Structure: Formed by transmembrane proteins claudins and occludins that create a "zipper-like" seal between adjacent cells
• Function:
  • Seal the intercellular space - prevent paracellular movement of ions and molecules (barrier function)
  • Maintain cell polarity by preventing diffusion of apical membrane proteins to basolateral surface
• Example: Intestinal epithelium, blood-brain barrier

2. Anchoring Junctions

• Proteins: E-cadherin (transmembrane) linked to actin filaments via catenins
• Form a belt below tight junctions
• Function: Cell-cell adhesion, tissue morphogenesis

• Proteins: Desmogleins/Desmocollins (cadherins) linked to intermediate filaments (keratin)
• Spot-like, rivet-like junctions
• Provide strong mechanical strength
• Example: Skin epidermis, cardiac muscle

• Attach epithelial cells to the basement membrane
• Use integrins linked to keratin intermediate filaments
• Example: Epithelium anchoring to lamina

• Integrins linking actin cytoskeleton to ECM fibronectin/laminin

3. Communicating Junctions (Gap Junctions)

• Structure: Formed by connexins (6 connexins form a connexon/hemichannel); two connexons from adjacent cells align to form a channel
• Function:
  • Allow direct passage of small molecules (<1000 Da) - ions, second messengers (cAMP, Ca²+), metabolites
  • Enable electrical coupling between cells
  • Enable metabolic cooperation
• Examples: Cardiac muscle (synchronized contraction), smooth muscle, hepatocytes

Q3 - Resting Membrane Potential (RMP) and its Genesis, with Gibbs-Donnan Equilibrium [10]

Definition of RMP

Ionic Basis of RMP

Genesis of RMP - Three Contributing Factors:

• The resting membrane is ~50-75x more permeable to K+ than Na+ (via leak channels)
• K+ diffuses out of the cell (down its concentration gradient)
• This leaves behind large negatively charged proteins (A-) that cannot cross the membrane
• Net result: inside becomes negative → this is the main source of the negative interior

• Pumps 3 Na+ out for every 2 K+ in (net removal of positive charge)
• Contributes approximately -4 mV to the RMP directly
• More importantly, maintains the ionic gradients that drive diffusion potentials
• Without this pump, gradients would dissipate and RMP would collapse

• Cl- distributes passively according to the electrical gradient
• Settles at its equilibrium potential determined by the existing voltage

Gibbs-Donnan Membrane Equilibrium

At equilibrium: [K+]in × [Cl-]in = [K+]out × [Cl-]out

1. Ion asymmetry: Diffusible cations (K+, Na+) are present in higher concentration on the side with the non-diffusible anions (inside)
2. Osmotic imbalance: More total particles inside → tendency for water to enter → cell swelling (prevented by Na+/K+ pump)
3. Electrical potential: A Donnan potential develops across the membrane (inside negative)
4. Cannot be a true equilibrium alone - cells would swell and lyse without the Na+/K+ pump counteracting osmotic forces

• Explains the unequal distribution of Cl- across the membrane
• Explains why intracellular fluid has higher colloid osmotic pressure
• The Na+/K+ pump acts as a "pump-leak model" to maintain a steady state (not true equilibrium) that prevents osmotic lysis
• The actual RMP is a steady state combining the Donnan potential + diffusion potentials + pump activity