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
All are complications of ulcerative colitis, except: 
Select and explain. 
A. Haemorrhage, B. Stricture, C. Malignant change, D. Fistula formation
D: Fistula formation
Answer: D: Fistula formation 
Explanation: Fistula formation is a hallmark feature of Crohn's , since the transmural inflammation is the norm in Crohn's disease. In ulcerative colitis it is always a paial thickness involvement and hence the fistula is not formed.