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

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

Answer: (C)

When Crohn's disease involves the terminal ileum, the nutrient that is most commonly deficient is vitamin B12 because absorption of B12 (with intrinsic factor) occurs specifically in that part of the small intestine. Inflammation or damage there disrupts the absorption pathway, leading to B12 deficiency over time. B12 stores in the body are large, so deficiency may take years to become evident, and it can cause macrocytic anemia and neurological symptoms like numbness or tingling. The other nutrients listed are not as directly tied to ileal absorption: iron is primarily absorbed earlier in the small intestine, while vitamin C deficiency isn’t specifically driven by ileal disease, and vitamin D deficiency relates more to fat malabsorption in general. Thus, B12 deficiency best fits the consequence of ileal involvement in Crohn's.