A 14-year-old boy presented with a 6-day history of an expanding ring of vesicles on the left forearm associated with focal burning. He had no history of trauma, insect bite, recently taking medication, or wearing jewelry in this area. His medical history was significant only for seasonal allergies. There was no family history of blistering disease.
On examination, we found a 6-cm annular bulla with deep-seated firm vesicles and a serous exudate on the volar aspect of the distal left forearm (Figure 1). The roof of the lesion was scraped, and microscopic examination of a potassium hydroxide preparation revealed abundant hyphae (Figure 2). Trichophyton tonsurans was isolated on fungal culture. Cultures for herpes simplex virus, varicella-zoster virus, and bacteria proved negative. The patient was treated with oral griseofulvin for 2 weeks and topical clotrimazole for 4 weeks, and his lesions resolved.