This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment & Nutritional Pathology, Subject name: Pathology
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? 
Select and explain. 
A. Vitamin A, B. Vitamin B12, C. Vitamin D, D. Vitamin E
B: Vitamin B12
Answer: B: Vitamin B12 
Explanation: 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