A 32-year-old woman is a vegan (i.e., a strict vegetarian who eats no animal products of any kind). She is weak and pale, and laboratory studies show a macrocytic, normochromic anemia (hemoglobin =6.2 g/dL). This patient most likely has which of the following vitamin deficiencies?

Vitamin B12 which is necessary for DNA synthesis, is contained only in animal products, including eggs. Extreme vegetarians may suffer vitamin B12 deficiency after many years of a restricted diet. The result is a macrocytic anemia similar to that seen in pernicious anemia. Macrocytic anemia is not a consequence of the other choices.Diagnosis: Vitamin B12 deficiency, megaloblastic anemia