In Crohn's disease, deficiency of which nutrient is most common due to ileal involvement?

• A. Calcium deficiency
• B. Iron deficiency
• C. Vitamin A deficiency
• D. B12 deficiency

Answer: (D)

When Crohn's disease involves the ileum, vitamin B12 absorption is the most affected. The terminal ileum is where the vitamin B12–intrinsic factor complex is reabsorbed, so disease or resection there leads to poor B12 uptake. Because the body stores B12 for years, deficiency develops slowly but is the most characteristic nutrient problem with ileal involvement. This can cause macrocytic anemia and neuropathy if not addressed.

Calcium deficiency can occur with fat malabsorption but is not as specifically linked to ileal disease. Iron deficiency is common in Crohn's due to chronic blood loss and inflammation, but iron is absorbed earlier in the gut (duodenum/proximal jejunum) rather than in the ileum. Vitamin A deficiency relates to fat-soluble vitamin absorption and can occur with general malabsorption, not uniquely with ileal Crohn's.