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': 'All of the following conditions are likely to be associated\nwith melaena except: \nSelect and explain. \nA. Car. of the stomach, B. Car. of the rectum, C. Chronic duodenal ulcer, D. Portal hypertension', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]