The Correct Answer is D: The Distal Ileum
The vitamin B12-intrinsic factor complex is absorbed exclusively in the distal ileum, making option D the correct answer. 1
Why D (The Distal Ileum) is Correct
Vitamin B12 (cobalamin) is selectively absorbed in the distal ileum when bound with gastric-derived intrinsic factor. 1 This is a fundamental principle of B12 physiology supported by multiple high-quality guidelines.
Key Physiologic Evidence:
Specific receptor localization: Cubilin and amnionless are the two receptor components that mediate uptake of the B12-intrinsic factor complex, and these receptors are present exclusively in the distal ileum 2
Anatomical distribution studies: IF-B12 receptors in human intestine are present in significant amounts in the entire distal three-fifths of intestine, with the highest concentration in the terminal ileum 3
Clinical confirmation: Resection of more than 20-30 cm of distal ileum puts patients at risk for B12 deficiency, confirming this is the sole absorption site 1, 4
Why the Other Options are Incorrect
A. The Stomach - INCORRECT
The stomach is where intrinsic factor is produced and secreted, NOT where B12 is absorbed. 5
- Gastric parietal cells produce intrinsic factor 5, 6
- In the stomach, B12 is released from food proteins by peptic digestion and initially binds to R-proteins (haptocorrin), not intrinsic factor 2, 5
- The stomach prepares B12 for absorption but does not absorb the B12-IF complex itself 2
B. The Duodenum - INCORRECT
The duodenum is where B12 transfers from R-proteins to intrinsic factor, but absorption does not occur here. 2, 5
- Pancreatic proteases degrade the B12-R-protein complex in the duodenum 2, 5
- The pH change in the duodenum triggers transfer of B12 to intrinsic factor 2
- This is a transfer site, not an absorption site—no IF-B12 receptors are present 3, 7
C. The Jejunum - INCORRECT
The jejunum lacks the specific receptors required for B12-intrinsic factor complex absorption. 3, 7
- Studies using homogeneous intrinsic factor demonstrate that IF-B12 is bound to ileal but NOT jejunal homogenates 7
- IF-B12 receptors are absent in the proximal small intestine including the jejunum 3
- The jejunum is involved in absorption of other nutrients but not B12 3
Clinical Relevance
Understanding this anatomy is critical for predicting B12 deficiency:
- Patients with Crohn's disease involving >20 cm of distal ileum require prophylactic monthly B12 supplementation (1 mg IM) indefinitely 4
- Resection of >30 cm of distal ileum will cause B12 deficiency requiring lifelong replacement 1
- Diseases affecting the terminal ileum (Crohn's disease, tuberculosis, lymphoma, radiation enteritis) impair B12 absorption 5
- Proximal small bowel disease (celiac disease affecting jejunum) does not directly impair B12 absorption unless the distal ileum is also involved 5