What is the most common form of the vitamin B12 deficiency?

• A. Pernicious anemia
• B. Iron deficiency anemia
• C. Folate deficiency anemia
• D. Aplastic anemia

Answer: (A)

B12 deficiency most commonly arises from impaired absorption due to loss of intrinsic factor, a condition known as pernicious anemia. Intrinsic factor is produced by gastric parietal cells and is essential for the timely absorption of dietary vitamin B12 in the terminal ileum. In pernicious anemia, autoimmune destruction of the parietal cells (often with autoimmune atrophic gastritis) leads to a shortage of intrinsic factor, so even with adequate B12 intake, absorption falls and deficiency develops. This makes pernicious anemia the leading cause of B12 deficiency in adults, especially in developed countries.

Other causes exist, such as very low dietary B12 intake (notably in strict vegans), ileal diseases or resection that blunt the absorption site, bacterial overgrowth, or pancreatic insufficiency, but these are less common. Clinically, B12 deficiency presents with macrocytic anemia and, if untreated, neurologic symptoms from demyelination. Lab clues include low B12 with elevated methylmalonic acid and homocysteine, and sometimes autoantibodies to intrinsic factor or parietal cells.