A 60-year-old woman presents for evaluation for chronic diarrhoea associated with faecal urgency. No fever, melena, haematochezia, abdominal pain, nausea, vomiting or weight loss. Diarrheal persists even during periods of fasting. Random colonic biopsies demonstrate mononuclear cell infiltration of the lamina propria and a thickened subepithelial collagen band as shown below. What is the most likely diagnosis?

**Core Concept**
The question requires the identification of a chronic diarrheal disorder characterized by a specific histopathological pattern, involving mononuclear cell infiltration and a thickened subepithelial collagen band. This is a classic presentation of collagenous colitis, a type of microscopic colitis.

**Why the Correct Answer is Right**
Collagenous colitis is characterized by chronic non-bloody diarrhea, often accompanied by abdominal pain and weight loss. The histopathological hallmark is the presence of a thickened subepithelial collagen band, which is visible on light microscopy. The mononuclear cell infiltration of the lamina propria is also a characteristic feature. The clinical presentation, histopathological findings, and absence of systemic symptoms like fever and weight loss make collagenous colitis the most likely diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ulcerative colitis (UC) typically presents with bloody diarrhea, mucosal ulcers, and a more pronounced inflammatory cell infiltrate in the lamina propria. The absence of these features makes UC less likely.

**Option B:** Irritable bowel syndrome (IBS) is a functional disorder characterized by abdominal pain and altered bowel habits, but it does not have a specific histopathological correlate.

**Option C:** Crohn's disease can present with chronic diarrhea, but it typically involves the formation of granulomas and a more pronounced inflammatory cell infiltrate, which is not seen in this patient.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing collagenous colitis is the recognition of the thickened subepithelial collagen band on histopathological examination. This feature is often not visible on routine staining and requires special stains like Masson's trichrome or reticulin staining.

**Correct Answer: C. Crohn's disease can present with chronic diarrhea, but it typically involves the formation of granulomas and a more pronounced inflammatory cell infiltrate, which is not seen in this patient.

✓ Correct Answer: C. Microscopic colitis