A 35-year-old woman who underwent resection of 50 cm of terminal ileum for Crohn's disease two years ago now has a macrocytic anemia with hypersegmented neutrophils. Serum vitamin B12 is 120 pg/mL. Schilling test shows low urinary excretion of radiolabelled B12 that is NOT corrected by adding intrinsic factor. What is the mechanism of the malabsorption?

• A Bacterial overgrowth consuming dietary B12 in the jejunum
• B Deficiency of gastric intrinsic factor secretion
• C Pancreatic enzyme deficiency preventing release of B12 from binding proteins
• D Loss of the ileal receptor sites for the intrinsic factor-B12 complex ✓

Explanation

The intrinsic factor-B12 complex is absorbed through cubilin receptors located exclusively in the terminal ileum. Resection of this segment removes the uptake sites, so exogenous intrinsic factor cannot correct the Schilling test, unlike in pernicious anemia where added intrinsic factor restores absorption. Such patients need lifelong parenteral vitamin B12 supplementation because oral crystalline B12 without ileum cannot be absorbed.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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