All of the following are features of Crohn’s disease except:

Explanation

## Correct Answer: A. Lead pipe appearance Lead pipe appearance is a classic radiological sign of **ulcerative colitis (UC)**, not Crohn's disease. This sign represents loss of normal mucosal haustra and a smooth, featureless colon that resembles a lead pipe on barium studies—a hallmark of chronic UC with severe mucosal atrophy. Crohn's disease, by contrast, typically shows a **"string sign"** (narrowed segment due to spasm and fibrosis) or **"cobblestone appearance"** (due to deep fissuring ulcers and intervening islands of regenerating mucosa). The question tests the critical distinction between UC and Crohn's radiological findings. In Indian clinical practice, this differentiation is essential for diagnosis, as both conditions present with chronic diarrhea and abdominal pain, but their management and prognosis differ significantly. Crohn's disease is characterized by transmural inflammation, skip lesions, and complications like fistulas and strictures—none of which are typical of UC. Lead pipe appearance exclusively reflects the end-stage mucosal damage pattern of UC. ## Why the other options are wrong **B. Transmural involvement** — Transmural inflammation is a **hallmark pathological feature of Crohn's disease**, distinguishing it from UC (which is limited to mucosa and submucosa). This transmural involvement leads to complications like fistulas, strictures, and abscess formation. This is a true feature of Crohn's, not an exception. **C. Perianal fistula** — Perianal fistulas are a **classic complication of Crohn's disease**, occurring in 20–30% of patients due to transmural inflammation and sinus tract formation. They are rare in UC and, when present, should raise suspicion for Crohn's disease. This is a true feature of Crohn's, not an exception. **D. Rectal sparing** — Rectal sparing (patchy involvement with skip lesions) is a **characteristic pattern of Crohn's disease**, reflecting its segmental, discontinuous nature. UC, by contrast, always involves the rectum and extends proximally in a continuous pattern. Rectal sparing is a true feature of Crohn's, not an exception. ## High-Yield Facts - **Lead pipe appearance** is pathognomonic for ulcerative colitis, not Crohn's disease—it reflects chronic mucosal atrophy and loss of haustra. - **Transmural involvement** in Crohn's disease (vs. mucosal-only in UC) explains why Crohn's causes fistulas, strictures, and abscess formation. - **Perianal fistulas** occur in ~25% of Crohn's patients and are rare in UC; their presence strongly suggests Crohn's disease. - **Skip lesions and rectal sparing** are hallmarks of Crohn's segmental inflammation; UC always involves rectum and extends continuously. - **String sign** (narrowed fibrotic segment) and **cobblestone appearance** (deep ulcers with regenerating islands) are radiological signatures of Crohn's disease. ## Mnemonics **UC vs Crohn's: Lead Pipe vs String** **Lead Pipe** = UC (smooth, featureless colon). **String Sign** = Crohn's (narrowed fibrotic segment). Remember: UC is uniform and continuous; Crohn's is patchy and deep. **Crohn's Complications: FFS** **F**istulas, **F**issures, **S**trictures—all due to transmural inflammation in Crohn's disease. UC lacks these because it's superficial. ## NBE Trap NBE exploits the fact that both UC and Crohn's are chronic inflammatory bowel diseases with overlapping clinical features (diarrhea, abdominal pain, weight loss). Students who conflate the two conditions may incorrectly assume lead pipe appearance applies to Crohn's, missing that it is UC-specific. ## Clinical Pearl In Indian tertiary care settings, when a patient with chronic diarrhea and perianal fistula presents, Crohn's disease should be the first diagnosis considered. Conversely, a patient with continuous bloody diarrhea, rectal involvement, and lead pipe appearance on imaging points to UC—a distinction that guides treatment (biologics vs. 5-ASA agents) and surgical planning. _Reference: Robbins Ch. 17 (Inflammatory Bowel Disease); Harrison Ch. 295 (Inflammatory Bowel Disease)_