A 28-year-old woman with Crohn's disease undergoes colonoscopy for evaluation of chronic diarrhea and weight loss. Biopsy shows non-caseating granulomas and transmural inflammation with skip lesions. Which is the most common site of involvement in Crohn's disease?

Question

Accepted Answer

B. Terminal ileum and cecum. ## Most Common Site of Crohn's Disease

**Key Point:** The terminal ileum and cecum (ileocecal region) are the most frequently involved sites in Crohn's disease, occurring in approximately 40–50% of all cases.

### Pattern of Involvement in Crohn's Disease

**High-Yield:** Crohn's disease can involve **any part of the GI tract from mouth to anus**, with a predilection for the ileocecal region.

### Frequency of Site Involvement

| Site | Frequency |
|------|----------|
| **Terminal ileum ± colon (ileocolic)** | 40–50% (most common) |
| **Small bowel alone** | 30% |
| **Colon alone** | 20% |
| **Stomach/duodenum** | 5% |
| **Rectum alone** | Rare |

### Why Terminal Ileum is Most Common

1. **Rich lymphoid tissue** — Peyer's patches are abundant in the terminal ileum
2. **Narrow lumen** — Creates a natural site of stasis and bacterial overgrowth
3. **Transition zone** — Between small and large bowel; altered microbiota
4. **Immunological factors** — Highest concentration of M cells and lymphoid follicles

### Distinguishing Features: Crohn's vs. Ulcerative Colitis

| Feature | Crohn's Disease | Ulcerative Colitis |
|---------|-----------------|-------------------|
| **Most common site** | Terminal ileum + cecum | Rectosigmoid |
| **Pattern** | Skip lesions (discontinuous) | Continuous |
| **Depth** | Transmural | Mucosa + submucosa |
| **Small bowel** | Frequently involved (40%) | Never |
| **Granulomas** | Present (non-caseating) in 30–50% | Absent |
| **Fistulas** | Common | Absent |

**Clinical Pearl:** Patients with Crohn's disease involving the terminal ileum often present with right lower quadrant pain (mimicking appendicitis) and may develop strictures or fistulas. Vitamin B₁₂ malabsorption is common due to terminal ileum involvement.

**Mnemonic:** **CRISP** — Crohn's involves the **R**ectum **I**nfrequently, **S**mall bowel **P**referentially (especially terminal ileum).

Answer

A. Descending and sigmoid colon

Answer

C. Jejunum

Answer

D. Rectum and sigmoid colon

A 28-year-old woman with Crohn's disease undergoes colonoscopy for evaluation of chronic diarrhea and weight loss. Biopsy shows non-caseating granulomas and transmural inflammation with skip lesions. Which is the most common site of involvement in Crohn's disease?

Explanation

Most Common Site of Crohn's Disease

Pattern of Involvement in Crohn's Disease

Frequency of Site Involvement

Why Terminal Ileum is Most Common

Distinguishing Features: Crohn's vs. Ulcerative Colitis

Practice similar questions

Join our NEET PG community

Site	Frequency
Terminal ileum ± colon (ileocolic)	40–50% (most common)
Small bowel alone	30%
Colon alone	20%
Stomach/duodenum	5%
Rectum alone	Rare

Feature	Crohn's Disease	Ulcerative Colitis
Most common site	Terminal ileum + cecum	Rectosigmoid
Pattern	Skip lesions (discontinuous)	Continuous
Depth	Transmural	Mucosa + submucosa
Small bowel	Frequently involved (40%)	Never
Granulomas	Present (non-caseating) in 30–50%	Absent
Fistulas	Common	Absent