This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Inflammatory Bowel Diseases, Subject name: Medicine
[{'content': 'Most common site of ulcerative colitis is? \nSelect and explain. \nA. Rectum, B. Caecum, C. Small intestine, D. Appendix', 'role': 'user'}, {'content': 'Answer: A \nExplanation: ANSWER: (A) RectumREF: Harrison s 18th ed ch: 295UC is a mucosal disease that usually involves the rectum and extends proximally to involve all or part of the colon. About 40-50% of patients have disease limited to the rectum and rectosigmoid, 30-40% have disease extending beyond the sigmoid but not involving the whole colon, and 20% have a total colitis', 'role': 'assistant'}]