A 28-year-old man with Crohn's disease is evaluated for a new complication. Imaging shows a fistulous tract between the terminal ileum and sigmoid colon with a surrounding abscess. Which single feature would be ABSENT if this patient instead had ulcerative colitis with the same clinical presentation?

Explanation

## Crohn's Disease Complications: Fistulas and Transmural Disease ### The Defining Pathologic Difference **Key Point:** Fistula and abscess formation are **pathognomonic for Crohn's disease** because they arise from **transmural inflammation**. Ulcerative colitis, being limited to mucosa and submucosa, cannot form fistulas or abscesses. ### Mechanism of Fistula Formation in Crohn's Disease ```mermaid flowchart TD A[Transmural inflammation in Crohn's]:::outcome --> B[Full-thickness bowel wall involvement]:::outcome B --> C[Penetrating ulcers breach serosa]:::action C --> D{Fistula tract formation}:::decision D -->|Adjacent bowel| E[Enteroenteric fistula]:::outcome D -->|Skin| F[Enterocutaneous fistula]:::outcome D -->|Abscess cavity| G[Fistulizing abscess]:::outcome H[Mucosal-only inflammation in UC]:::outcome --> I[No transmural breach]:::action I --> J[No fistula formation possible]:::action ``` ### Comparison of Complications | Complication | Crohn's Disease | Ulcerative Colitis | |--------------|-----------------|-------------------| | **Fistulas** | Common (20–40%) | Absent | | **Abscesses** | Common | Absent | | **Strictures** | Common (transmural fibrosis) | Rare | | **Toxic megacolon** | Rare | Possible | | **Colorectal cancer** | Increased risk | Increased risk | | **Systemic inflammation** | Present (elevated CRP) | Present (elevated CRP) | ### Why Other Options Are Not Discriminatory **High-Yield:** Both UC and Crohn's disease: - Elevate inflammatory markers (CRP, ESR) due to mucosal inflammation - Respond to 5-ASA compounds (though Crohn's response is weaker) - Carry increased colorectal cancer risk (UC slightly higher absolute risk) **Clinical Pearl:** The presence of fistulas, abscesses, or strictures in a patient with IBD is virtually diagnostic of Crohn's disease. Their absence does not exclude Crohn's, but their presence excludes UC. **Warning:** Do not confuse "fistula formation" with "fistulizing disease." Some Crohn's patients have non-fistulizing disease (inflammatory or stricturing phenotype); however, fistulas themselves cannot occur in UC under any circumstance. [cite:Harrison 21e Ch 295]