An endoscopy is performed on a 28-year-old man with a 3-year history of crampy right lower quadrant pain, chronic diarrhea, and weight loss. The endoscopic findings shown in the diagram at **A** reveal skip lesions, aphthous ulcers, deep linear ulcers, and cobblestone mucosa with rectal sparing. The terminal ileum is intubated and shows similar changes. Which of the following BEST explains the pathogenesis of the mucosal inflammation pattern observed at **A**?

Explanation

## Why option 1 is right The endoscopic findings at **A** — skip lesions, aphthous ulcers, deep linear ulcers, cobblestone mucosa, and rectal sparing with terminal ileal involvement — are pathognomonic for Crohn disease. The clinical anchor is that CD is a chronic transmural granulomatous inflammatory bowel disease driven by a dysregulated Th1- and Th17-skewed mucosal immune response to luminal microbiota in a genetically susceptible host. The strongest single genetic risk factor is NOD2/CARD15 polymorphisms, which encode defective bacterial sensing mechanisms. This explains both the discontinuous (skip) pattern of inflammation and the transmural depth of ulceration seen at **A**. (Sleisenger & Fordtran 11e Ch 116; Harrison 21e Ch 326) ## Why each distractor is wrong - **Option 2**: Describes the pathogenesis of ulcerative colitis — continuous inflammation from rectum proximally without skip lesions. UC does not spare the rectum and lacks the discontinuous pattern seen at **A**. This is a common confusion point but contradicts the rectal sparing and skip lesions documented in the diagram. - **Option 3**: Describes ischemic colitis, which presents with segmental edema and submucosal hemorrhage at watershed vascular zones (splenic flexure, rectosigmoid junction). The endoscopic appearance and clinical context (young patient, 3-year chronic course) do not fit acute ischemic injury; ischemic colitis is typically acute in elderly patients with vascular risk factors. - **Option 4**: Describes pseudomembranous colitis caused by *Clostridioides difficile* toxin, characterized by yellow-white adherent plaques. This is an acute infectious colitis, not a chronic granulomatous disease, and lacks the skip lesions and cobblestoning pattern at **A**. It is also not associated with terminal ileal involvement. **High-Yield:** Skip lesions + rectal sparing + terminal ileal involvement + cobblestoning + NOD2 mutations = Crohn disease (NOT ulcerative colitis). [cite: Sleisenger & Fordtran 11e Ch 116; Harrison 21e Ch 326]