This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Inflammatory bowel disease, Subject name: Surgery
[{'content': 'Ulcerative colitis stas from: \nSelect and explain. \nA. Rectum, B. Sigmoid colon, C. Ascending colon, D. Any pa', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Rectal involvement (proctitis) is the hallmark of the disease, and the diagnosis should be seriously questioned if the rectal mucosa is not affected.', 'role': 'assistant'}]