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': '65-year-old man presents with anaemia and posterior column dysfunction, the likely cause is - \nSelect and explain. \nA. B1-deficit, B. B12-deficit, C. SSPE, D. Multiple selerosis', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]