NBME Form 16 Practice Test 2026 - Free NBME Practice Questions and Study Guide

Intrinsic factor produced by gastric parietal cells is essential for the absorption of vitamin B12. When parietal cells are destroyed, intrinsic factor is lacking, so vitamin B12 cannot be efficiently absorbed in the ileum, leading to a deficiency. Vitamin B12 is crucial for DNA synthesis and proper myelin formation, and its deficiency causes megaloblastic anemia and neurologic symptoms. The other vitamins listed do not depend on intrinsic factor for absorption—vitamin C, vitamin D, and vitamin K are absorbed through different pathways (not IF-dependent)—so their deficiencies aren’t caused by parietal cell destruction.