B12 Deficiency Post-Ileal Resection — RACP Adult Medicine MCQ

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EasyGeneral Internal MedicineB12 Deficiency Post-Ileal ResectionRACP Adult Medicine

A 50-year-old man with Crohn's disease has a terminal ileum resection 15 years ago. He presents with macrocytic anaemia (Hb 95 g/L, MCV 108 fL) and bilateral lower limb paraesthesiae. His vitamin B12 is 85 pmol/L (low). What is the cause of his B12 deficiency?

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Correct answer: A — Dietary insufficiency

Vitamin B12 is absorbed exclusively in the terminal ileum via intrinsic factor-mediated receptors (cubam receptor complex). Surgical resection of the terminal ileum (a common procedure in Crohn's disease for stricturing or penetrating disease) permanently eliminates the B12 absorption site. These patients require lifelong parenteral B12 replacement (IM hydroxocobalamin 1 mg every 3 months – oral supplementation is inadequate as absorption is not possible without a terminal ileum). This should be proactively commenced post-ileal resection, not awaited until deficiency develops.

Reference: eTG – 2025 – Haematology; eTG – 2025 – Gastrointestinal