A 24-year-old male presents with a 6-month history of chronic diarrhea, right lower quadrant abdominal pain, and weight loss. Ileocolonoscopy reveals the endoscopic appearance marked as **A** in the diagram — deep linear ulcers intersecting to create islands of edematous mucosa, with sharply demarcated normal segments interspersed between inflamed areas. Which of the following BEST describes the pathophysiological basis for the segmental pattern of inflammation seen in this patient's disease?

Question

Accepted Answer

C. Dysregulated mucosal immunity in genetically susceptible individuals triggered by environmental factors and gut dysbiosis, resulting in transmural inflammation with skip lesions. ## Why option 1 is right

The cobblestone appearance marked **A** — with deep linear ulcers creating islands of edematous mucosa and sharply demarcated normal (skip) segments — is the endoscopic hallmark of Crohn disease. This segmental, discontinuous pattern of inflammation is a cardinal distinguishing feature of CD and directly reflects the underlying pathophysiology: dysregulated mucosal immunity in genetically susceptible individuals (NOD2/CARD15, ATG16L1, IL23R mutations) triggered by environmental factors and gut dysbiosis. The transmural nature of CD inflammation (extending through all bowel wall layers) explains the deep linear ulcers and fissures, and the skip lesions represent the discontinuous, segmental involvement characteristic of CD. This pathophysiology is explicitly stated in ECCO and ACG guidelines and is the fundamental mechanism distinguishing CD from other IBDs.

## Why each distractor is wrong

- **Option 2**: Describes ulcerative colitis, which presents with continuous diffuse mucosal inflammation limited to the colon and rectum, WITHOUT skip lesions, transmural involvement, or the cobblestone pattern. UC does not involve the terminal ileum or proximal small bowel.
- **Option 3**: Describes pseudomembranous colitis caused by *Clostridioides difficile*, which presents with yellow plaques (marked **D** in the diagram), not cobblestone mucosa. This is an infectious etiology, not the immune-mediated dysregulation of CD.
- **Option 4**: Describes superficial ulcerative colitis-type inflammation limited to the lamina propria. CD is fundamentally transmural, with inflammation extending into the muscularis propria and beyond, creating the deep fissures and linear ulcers that form the cobblestone pattern.

**High-Yield:** Crohn disease = transmural + segmental (skip lesions) + cobblestone + deep linear ulcers; ulcerative colitis = mucosal-only + continuous + diffuse erythema.

[cite: ECCO Crohn Disease Guideline 2022; ACG Crohn Disease Guideline 2018]

Answer

A. Continuous diffuse mucosal inflammation limited to the colon, sparing the terminal ileum and proximal small bowel

Answer

B. Pseudomembranous infection with Clostridioides difficile toxins causing focal plaques and mucosal necrosis

Answer

D. Superficial mucosal ulceration confined to the lamina propria without transmural involvement or fissuring

Explanation

Why option 1 is right

Why each distractor is wrong

Practice similar questions