A healthy 36-year-old man was referred to the dermatology clinic for a 5-day history of an exquisitely tender rash at the base of his left thumb.

One day prior to presentation, he went to a local emergency department, where he was prescribed clindamycin and valacyclovir. The patient works as a plumber and attributed his tender rash to a “spider bite” he had incurred while working in an old building. The patient was afebrile, and physical examination revealed several vesicles with purulent fluid on an erythematous base, at the base of the left thumb.

There was no lymphadenopathy, lymphangitis, or mucous membrane involvement. The rash was not found elsewhere on the body. Vesicular fluid was collected and sent for further analysis.