## Crohn's Disease vs. Ulcerative Colitis: Complications and Features ### Comparative Pathophysiology and Complications **Key Point:** While Crohn's disease is transmural and prone to fistulization and strictures, **toxic megacolon is actually MORE common in UC than in Crohn's disease**, making this the false statement. | Feature | Crohn's Disease | Ulcerative Colitis | Reason | |---------|-----------------|-------------------|--------| | Transmural inflammation | Yes | No (mucosal/submucosal) | Crohn's affects all layers | | Fistulization | Common (20–30%) | Rare | Transmural involvement allows fistula tracts | | Stricture formation | Common | Rare | Fibrosis in transmural disease | | Toxic megacolon | Less common | **More common** | UC causes acute severe mucosal inflammation; Crohn's is more chronic/fibrotic | | Perianal disease | 30% of patients | Absent | Anorectal involvement is hallmark of Crohn's | | Small bowel involvement | Common | Never | Crohn's affects any part of GI tract | ### Why Toxic Megacolon Favors UC **High-Yield:** Toxic megacolon develops from: 1. Acute, severe mucosal inflammation 2. Loss of colonic muscle tone 3. Transmural necrosis with perforation risk UC causes **acute, fulminant mucosal disease** with rapid loss of muscle function. Crohn's, by contrast, is typically **chronic and transmural with fibrosis**, which actually **prevents** acute dilation by creating rigid, strictured segments. **Clinical Pearl:** A patient with UC in fulminant relapse is at much higher risk of toxic megacolon (especially if given antiperistaltic agents or opioids). Crohn's patients are more likely to develop obstructive symptoms from strictures than acute megacolon. ### The Three Correct Statements 1. **Fistulizing disease** — Present in 20–30% of Crohn's; rare in UC (transmural involvement) 2. **Stricture formation** — Hallmark of Crohn's; due to fibrosis and inflammation; rare in UC 3. **Perianal disease** — Fistulas, abscesses, skin tags in ~30% of Crohn's; absent in UC **Mnemonic:** **FIST** = Features of Crohn's: **F**istulas, **I**nvolvement (transmural), **S**trictures, **T**ransmural disease. Toxic megacolon is NOT a Crohn's hallmark — it's a UC complication. ### Exam Strategy **Tip:** When comparing IBD complications, remember: - **Crohn's** → fistulas, strictures, perianal disease, small bowel involvement - **UC** → toxic megacolon, continuous colonic involvement, no fistulas, no perianal disease
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