Mild thickening of the mucosa in pseudomembranous colitis, on the right the corresponding image from the endoscopy.
A pseudo membrane is the consequence of local inflammation which causes yellowish-white plaques that are attached to the mucose and consist of fibrin and cell detritus (necrotic epithelial cells and leukocytes).
Pseudomembranes are a characteristic symptom of different infectious diseases, for example:
Source: Ultraschall im Netz