Which is the key distinguishing feature between Crohn's disease and ulcerative colitis?

• A. Crohn's affects entire bowel wall thickness; ulcerative colitis is limited to the colon lining
• B. Crohn's is limited to the colon; ulcerative colitis affects the entire bowel wall
• C. Crohn's only affects the small intestine; ulcerative colitis affects the colon
• D. They both affect only the mucosa

Answer: (A)

The main point is how deeply the inflammation involves the bowel wall. Crohn's disease features transmural inflammation, meaning the entire thickness of the bowel wall can be affected, and it can occur anywhere along the GI tract with patchy involvement and potential fistulas or strictures. Ulcerative colitis, by contrast, is limited to the mucosa (and submucosa) of the colon, starting at the rectum and spreading in a continuous fashion. This depth difference is why the description of full-thickness involvement in Crohn's and mucosal lining involvement in ulcerative colitis best captures the key distinction.