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 50-year-old woman presents with easy fatigability, a smooth sore tongue, numbness and tingling of the feet, and weakness of the legs. A complete blood count shows a megaloblastic anemia that is not reversed by folate therapy. Hemoglobin is 5.6 g/dL, WBC count is 5,100/mL, and platelets are 240,000/mL. This patient most likely has a deficiency of which of the following vitamins? 
Select and explain. 
A. Vitamin B1 (thiamine), B. Vitamin B2 (riboflavin), C. Vitamin B12, D. Vitamin K
C: Vitamin B12
Answer: C: Vitamin B12 
Explanation: Except for a few rare situations, vitamin B12 (cyanocobalamin) deficiency is usually a result of pernicious anemia, an autoimmune disease of the stomach. Vitamin B12 is required for DNA synthesis, and its deficiency results in large (megaloblastic) nuclei.Diagnosis: Vitamin B12 deficiency, pernicious anemia