A 28-year-old woman with a 2-year history of chronic diarrhea, right lower quadrant pain, weight loss, and oral aphthae undergoes ileocolonoscopy. The procedure reveals a tight asymmetric stricture in the terminal ileum with proximal pre-stenotic dilation, cobblestone mucosa, and deep linear ulcerations. Biopsies show transmural inflammation with non-caseating granulomas. The structure marked **A** in the diagram represents the tight luminal stricture. Which of the following best describes the pathophysiological mechanism underlying the formation of this stricture in Crohn disease?

Explanation

## Why option 1 is correct The structure marked **A** — the tight luminal stricture — is a hallmark of fibrostenotic Crohn disease (Montreal classification B2 behavior). The ECCO Crohn Guidelines 2022 and standard pathophysiology establish that strictures in Crohn disease result from **chronic transmural granulomatous inflammation** driven by dysregulated mucosal immunity (NOD2/CARD15 mutations increase risk 2–4×) with **Th1/Th17-mediated TNF-α-dependent tissue injury**. This chronic inflammatory process causes repeated cycles of ulceration, healing, and fibrosis, ultimately leading to the fibrotic stricture seen in the diagram. The presence of non-caseating granulomas on biopsy confirms the granulomatous nature of the disease. ## Why each distractor is wrong - **Option 2 (acute bacterial-driven stricture)**: Crohn strictures are NOT acute phenomena caused by bacterial invasion. They develop over months to years through chronic inflammatory remodeling, not acute infection. Acute inflammation would present with acute obstruction, not the chronic pre-stenotic dilation seen here. - **Option 3 (mucosal-only inflammation)**: This describes ulcerative colitis, which is limited to mucosa and submucosa and does NOT cause strictures. Crohn disease is **transmural** — involving all layers of the bowel wall — which is essential for stricture formation. The transmural nature is confirmed by the biopsy findings. - **Option 4 (caseating granulomas with AFB)**: This describes tuberculosis, a critical differential diagnosis in the Indian context (especially with terminal ileal disease). TB causes **caseating granulomas** with acid-fast bacilli, whereas Crohn disease causes **non-caseating granulomas** without organisms. The biopsy explicitly shows non-caseating granulomas, ruling out TB. **High-Yield:** Crohn strictures = chronic transmural Th1/Th17-TNF-α inflammation → fibrosis; non-caseating granomas = Crohn; caseating granomas + AFB = TB (always rule out in India with terminal ileal disease). [cite: ECCO Crohn Guidelines 2022; Montreal Classification; Textbook of Gastroenterology]