American Board of Internal Medicine (ABIM) Certification Practice Exam

What are the histological subtypes of microscopic colitis?

1. Ulcerative and Crohn's colitis

2. Lymphocytic and collagenous colitis

3. Focal and diffuse colitis

4. Granulomatous and pseudomembranous colitis

The correct answer is: Lymphocytic and collagenous colitis

Microscopic colitis is characterized by chronic, non-bloody diarrhea and is distinguished histologically into two primary subtypes: lymphocytic colitis and collagenous colitis. Lymphocytic colitis is characterized by an increase in intraepithelial lymphocytes, typically with relatively normal colonic architecture, while collagenous colitis is marked by a thickened collagen band beneath the surface epithelium, accompanied by an inflammatory infiltrate. Both subtypes share similar clinical manifestations, but their distinct histological features are critical for diagnosis. This distinction is essential for appropriate management and understanding patient outcomes, as each subtype may respond differently to treatments. The presence of increased intraepithelial lymphocytes in lymphocytic colitis and the collagenous layer in collagenous colitis provides a clear basis for their classification as the specific histological subtypes of microscopic colitis.