Pyoderma gangrenosum ((a) gross appearance of left lower thigh, (b) H&E). A demarcated deep ulcer, 4 cm in size, is observed. Fibrinous exudation is seen at the ulcer base, and erythematous change is associated around the ulcer (a). Microscopic features of the first biopsy specimen are nonspecific. Ulcer base is composed of fibrinous exudation and granulation tissue (b). Neither granulomatous reactions nor infectious agents are identified.