Large asymmetric ulceration with yellow fibrinous debris centrally and bright-red to violaceous inflamed and undermined border peripherally, characteristic of pyoderma gangrenosum.

The pustule or papule will break down, evolving into erosions/ulcers with multiple crater-like holes. These ulcerated plaques consist of small fistulous tracks from which drainage occurs.

The legs are the most common site. New lesions form at sites of injury in a phenomenon called pathergy. Multiple lesions are typical.

Differentiation from other diseases causing ulcers is sometimes very difficult. Malignancies may present as ulcerations with exactly the same appearance; biopsy uncertain.