In Crohn's disease with terminal ileum involvement, a deficiency of which vitamin is likely?

• A. Vitamin C
• B. Vitamin B12
• C. Vitamin D
• D. Vitamin K

Answer: (B)

The important concept is that vitamin B12 absorption depends on the terminal ileum, where the vitamin B12–intrinsic factor complex is absorbed. When Crohn’s disease involves the terminal ileum, this absorption is impaired, leading to vitamin B12 deficiency. B12 deficiency causes macrocytic (megaloblastic) anemia and can produce neurologic symptoms such as paresthesias or gait disturbances from demyelination, with stores in the body lasting years before symptoms appear. Folate absorption occurs earlier in the small intestine, so Crohn’s ileal involvement doesn’t typically cause folate deficiency as the primary issue. Fat-soluble vitamins (A, D, E, K) can be affected if fat absorption is impaired, but the classic and most direct deficiency in ileal Crohn’s is vitamin B12.