A painful nodule with induration and spreading erythema

Elizabeth Race, MD, MPH, Cindy Berthelot, MS, and Jennifer Clay Cather, MD

A young woman presented with a 2-day history of a 2- to 3-cm erythematous painful papule on her right flank, which she thought was a spider bite. Initially, the lesion was a nodule that was warm, tender, and fluctuant on palpation. Clinically the lesion was most consistent with an abscess, spider bite, or inflamed cyst. The lesion was incised, drained, and cultured. Empirically therapy with cephalexin was started. Within 24 hours, the patient presented to the dermatology clinic with a low-grade fever (38.3°C, 101°F), and the lesion had become more tender. The erythema had spread to 20 cm, and the central induration had spread to 9 cm.

What is the most likely diagnosis? What is the most appropriate therapy at this point?