Which vitamin deficiency underlies pernicious anemia?

• A. Vitamin B2 deficiency
• B. Vitamin B12 deficiency due to intrinsic factor deficiency
• C. Vitamin B6 deficiency
• D. Folate deficiency

Answer: (B)

Pernicious anemia occurs when vitamin B12 cannot be absorbed because intrinsic factor is deficient, usually from autoimmune destruction of gastric parietal cells. Without intrinsic factor, B12 cannot be effectively absorbed in the terminal ileum, leading to a true B12 deficiency. Vitamin B12 is essential for DNA synthesis in red blood cell production and for maintaining myelin, so its deficiency causes megaloblastic anemia and, sometimes, neurologic symptoms. This specific cause—B12 deficiency due to lack of intrinsic factor—distinguishes pernicious anemia from other deficiencies like riboflavin, B6, or folate. Folate deficiency can also cause megaloblastic anemia, but pernicious anemia is defined by the impaired B12 absorption driven by intrinsic factor deficiency.