Endoscopic and pathologic evaluation of the sigmoid colon on day 525 after transplantation. (A) Endoscopic view of the colon shows serpiginous ulceration of the mucosal surface (arrow). (B) Low-power view of colon biopsy shows atypical glands indicating regeneration, intramural bacteria (red arrow), fibrinous debris coating denuded luminal surface (green arrow), and an exploded crypt lacking viable cells (blue arrow). Hematoxylin and eosin, original magnification, ×5. (C) High-power view shows crypts with apoptotic cells (arrows) and intense inflammatory infiltrate surrounding the crypts. Hematoxylin and eosin, original magnification, ×20.