Vitamin B₁₂ absorption in the terminal ileum requires binding to intrinsic factor (IF). The IF–B₁₂ complex binds which specific receptor on ileal enterocytes for receptor-mediated endocytosis?

• A Megalin (LRP2) acting alone on ileal brush border
• B ABCD4 transporter on the apical membrane of enterocytes
• C Cubilin (receptor for IF-B₁₂ complex) acting in concert with amnionless (AMN) as a co-receptor ✓
• D Transcobalamin II receptor (TCblR/CD320) on ileal apical membrane

Explanation

The IF–B₁₂ complex in the distal ileum binds to cubilin, a large endocytic receptor that lacks a transmembrane domain and requires amnionless (AMN) as a co-receptor for proper membrane anchoring and endocytosis. The cubilin–AMN complex (also called CUBAM) mediates internalization of IF–B₁₂ into enterocytes. Mutations in cubilin or amnionless cause Imerslund-Gräsbeck syndrome (juvenile megaloblastic anemia with proteinuria). Megalin is expressed in proximal tubular cells and is involved in filtered B₁₂ retrieval. TCblR/CD320 binds transcobalamin II–B₁₂ complex (not IF–B₁₂) for delivery to all body cells.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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