A 35-year-old with Crohn's disease has ileal resection and develops neurological symptoms: subacute combined degeneration of the cord with megaloblastic anaemia. Vitamin B12 absorption requires intrinsic factor binding in the stomach and absorption at the:

• A Duodenum, via brush-border divalent metal transporter (DMT-1)
• B Jejunum, via multidrug resistance-associated protein 2 (MRP2)
• C Proximal ileum, via ASBT (apical sodium-dependent bile acid transporter)
• D Terminal ileum, via cubilin-amnionless receptors ✓

Explanation

The B12-IF complex is absorbed exclusively in the terminal ileum through cubilin (a peripheral membrane protein) and amnionless (CUBN-AMN complex), which internalise the complex via receptor-mediated endocytosis. Resection of the terminal ileum — as in Crohn's disease or surgical resection — abolishes B12 absorption, causing deficiency after 3–5 years (body stores last that long). This requires lifelong intramuscular B12 supplementation (bypassing the enteral route entirely).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.