American Board of Internal Medicine (ABIM) Certification Practice Exam 2026 - Free ABIM Practice Questions and Study Guide

The histological feature that is primarily distinguished in collagenous colitis is the thickening of the subepithelial collagen band. This condition is characterized by a distinct histopathological finding where there is a layer of collagen that becomes notably thickened beneath the epithelium, which can be observed in tissue samples under a microscope. This band of collagen, often more than 10 micrometers thick, is a crucial diagnostic criterion for collagenous colitis and differentiates it from other forms of colitis.

In collagenous colitis, although intraepithelial lymphocytosis can be seen, it does not serve as the hallmark feature of the disease. Similarly, while the presence of abscesses and crypt distortion may occur in other forms of colitis, such as ulcerative colitis or infectious colitis, they are not characteristic of collagenous colitis itself. Focusing on the thickened collagen band is essential for diagnosing this type of colitis and understanding its pathophysiology.