Intrinsic factor–B12 complex is taken up by receptors in the

• A. Proximal jejunum
• B. Distal ileum
• C. Colon
• D. Esophagus

Answer: (B)

The essential point is that B12 absorption requires a specific receptor-mediated process that happens only in the distal ileum. After B12 binds intrinsic factor in the stomach, the IF–B12 complex travels to the ileum, where enterocytes express a receptor complex (cubilin with amnionless) that binds the complex and drives endocytosis. This makes the distal ileum the site of active, receptor-mediated uptake of IF–B12.

Other regions don’t have this specialized receptor system, so the proximal jejunum, colon, and esophagus do not efficiently absorb the IF–B12 complex. If the distal ileum is damaged or removed, B12 absorption drops significantly, leading to deficiency.