A 55-year-old man who underwent total gastrectomy for gastric carcinoma five years ago returns with pallor, glossitis, and paresthesias. Hemoglobin is 7.8 g/dL with macrocytosis and hypersegmented neutrophils. The step in vitamin B12 assimilation lost by his surgery is:
- A Proteolytic release of B12 from food proteins by pepsin
- B Uptake of the B12-intrinsic factor complex by ileal receptors
- C Binding of B12 to intrinsic factor secreted by parietal cells ✓
- D Transfer of B12 from haptocorrin to transcobalamin II in plasma
Correct answer: C. Binding of B12 to intrinsic factor secreted by parietal cells
Explanation
Intrinsic factor is secreted by gastric parietal cells, so total gastrectomy abolishes its supply entirely. Dietary C12 then binds haptocorrin but never forms the IF-C12 complex required for recognition by cubam receptors in the terminal ileum, leading to deficiency after hepatic stores are exhausted over three to five years. Ileal uptake and transcobalamin II transfer remain structurally intact, and although pepsin release is also lost, the absence of intrinsic factor is the decisive irreversible defect.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.