This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'A man who had undergone total gastrectomy remains well for five years. Then he develops anaemia. What is the most likely cause of this anaemia ? \nSelect and explain. \nA. Protein deficiency, B. Zinc deficiency, C. Folic acid deficiency, D. Vitamin B12 deficiency', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]