A 20-year-old man presented with a mildly pruritic, progressive rash that began on his scalp and spread to his neck, back, chest, and thighs. A trial of oral cephalexin was not helpful. He denied other medication use, travel, or illicit drug use. Examination revealed numerous grouped vesicles and bullae coalescing into annular plaques, on an erythematous base with central clearing and scale. The patient was afebrile and appeared well. What would you do next?