## Why option 1 is right Skip lesions (marked **A**) are the pathognomonic segmental discontinuous areas of inflammation in Crohn disease, with normal bowel segments interspersed between affected areas. This pattern reflects the transmural (full-thickness) nature of Crohn inflammation. In contrast, ulcerative colitis shows continuous inflammation limited to the mucosa and submucosa, without skip lesions. This distinction is fundamental to differentiating the two inflammatory bowel diseases and is a key diagnostic criterion on colonoscopy and biopsy (Robbins 10e, Ch 17). ## Why each distractor is wrong - **Option 2**: While non-caseating granulomas are present in 30–50% of Crohn disease biopsies, they are not the primary mechanism creating skip lesions. Ulcerative colitis does not have caseating granulomas; granulomas in IBD are non-caseating by definition. The skip lesion pattern is due to transmural inflammation, not granuloma distribution. - **Option 3**: Smoking worsens Crohn disease (a risk factor for progression), but it does not cause skip lesions. Smoking is actually protective in ulcerative colitis, not the reverse. Skip lesions are an intrinsic feature of Crohn pathophysiology, not a smoking-induced phenomenon. - **Option 4**: This reverses the anatomical distribution. Crohn disease can affect any part of the GI tract from mouth to anus (most commonly terminal ileum), whereas ulcerative colitis is limited to the colon and rectum. Skip lesions occur throughout the Crohn-affected bowel, not just the colon. **High-Yield:** Skip lesions = segmental discontinuous inflammation = Crohn disease; continuous inflammation = ulcerative colitis. This endoscopic/histological finding is the single most useful discriminator between the two IBDs. [cite: Robbins 10e Ch 17]
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