## Why "Transmural inflammation with submucosal edema creating islands of residual mucosa between deep ulcers" is right The cobblestone appearance marked **A** is the hallmark endoscopic finding of Crohn disease and results directly from transmural (full-thickness) inflammation. Deep linear and transverse ulcers penetrate through the mucosa and submucosa, while intervening areas of edematous mucosa remain elevated, creating the characteristic "cobblestone" or "mosaic" pattern. This appearance is pathognomonic for Crohn disease and distinguishes it from ulcerative colitis, which shows continuous mucosal involvement without islands of preserved mucosa. (Harrison 21e Ch 326) ## Why each distractor is wrong - **Superficial mucosal ulceration limited to the lamina propria with preserved muscularis mucosae**: This describes ulcerative colitis, not Crohn disease. UC is limited to mucosa and submucosa and produces a continuous pattern of inflammation, not cobblestone appearance. - **Circumferential ulceration with stricture formation and luminal narrowing**: While strictures do occur in Crohn disease from transmural fibrosis, this describes the mechanism of stricture formation, not the cobblestone mucosal appearance itself. - **Crypt distortion and goblet cell depletion with continuous mucosal involvement**: This describes the histologic pattern of ulcerative colitis with continuous involvement, not the transmural pattern that creates cobblestone mucosa. **High-Yield:** Cobblestone mucosa = transmural inflammation + edematous islands between deep ulcers = Crohn disease; continuous mucosal ulceration = ulcerative colitis. [cite: Harrison 21e Ch 326]
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