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
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 ? 
Select and explain. 
A. Protein deficiency, B. Zinc deficiency, C. Folic acid deficiency, D. Vitamin B12 deficiency
D: Vitamin B12 deficiency
Answer: D: Vitamin B12 deficiency 
Explanation: None