What is the primary histopathologic difference between Crohn's disease and ulcerative colitis?

• A. Transmural inflammation with skip lesions and noncaseating granulomas
• B. Continuous mucosal/submucosal inflammation starting in the rectum
• C. Pseudomembrane formation overlying the mucosa
• D. Epithelial dysplasia restricted to the colon

Answer: (A)

The key idea is how inflammation is distributed and how deep it goes in the bowel wall. Crohn's disease commonly shows inflammation that goes through the full thickness of the bowel wall (transmural), with patchy areas of disease separated by normal tissue (skip lesions). Noncaseating granulomas may be present, reflecting a granulomatous reaction. This combination explains why Crohn's often affects any part of the GI tract, can cause fistulas, and produces patchy disease.

In contrast, ulcerative colitis features inflammation that is continuous, starting in the rectum and extending proximally, and it stays limited to the mucosa and submucosa rather than involving all layers. The distribution is uniform rather than patchy, and granulomas are not a typical feature.

The other options describe patterns not central to distinguishing Crohn's from UC: pseudomembrane formation is classically seen with C. difficile infection, not as a defining feature of Crohn's vs UC; epithelial dysplasia relates to cancer risk in chronic colitis (especially UC) but is not the primary histologic distinction between the two diseases.