Clinical featuresPG most commonly occurs on the lower extremities, although other areas, including the trunk, abdomen, scalp and face, may be affected. It may also affect extra-cutaneous locations, including the lungs, eyes and mouth (termed pyostomatitis vegetans) 3 . Up to 25% of patients affected with PG will experience 'pathergy' or the Koebner phenomenon 4 . This occurs when localised trauma causes worsening of existing PG or the formation of new PG lesions. The mechanism underlying