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: Medicine
Invariably involved site in ulcerative colitis; 
Select and explain. 
A. Sigmoid colon, B. Transverse colon, C. Ileum, D. Rectum
D: Rectum
Answer: D: Rectum 
Explanation: Most common site involved in ulcerative colitis is the rectum. Rectum is usually spared in Crohn's disease.