Pyoderma Gangrenosum Mimicking Necrotizing Fasciitis: A Case Report.

Pyoderma gangrenosum (PG) is a rare neutrophilic dermatosis that can mimic other severe conditions, complicating diagnosis. We report a case of PG of the knee initially mistaken for necrotizing fasciitis (NF) in a 68-year-old woman who presented with knee swelling, erythema, papules, and pustules following a fall. Despite antibiotic therapy and surgical debridement for suspected NF, the patient's fever persisted, and laboratory markers (elevated WBC and C-reactive protein) did not improve. Histopathology revealed a dense neutrophilic infiltrate, and negative cultures ruled out infection, leading to a PG diagnosis. Treatment with oral prednisolone (60 mg/day) rapidly resolved symptoms, followed by negative pressure wound therapy, skin grafting, and adalimumab initiation. The patient stabilized without relapse. Key diagnostic clues included abundant pustules, characteristic of pustular PG, and the absence of fat tissue necrosis during surgery. This case highlights the diagnostic challenge of distinguishing PG from NF, as both may involve deep tissue inflammation. While histopathology and cultures are critical, results are delayed, necessitating reliance on clinical observations. Careful assessment of skin lesions and evaluation of fat necrosis (finger test) can prevent unnecessary surgical interventions, emphasizing the importance of considering PG in atypical presentations of suspected NF.