A 71-year-old woman presents to the emergency department for evaluation of a blistering, intensely pruritic generalized rash that started 5 days earlier. Multiple ruptured and intact hemorrhagic bullae are obvious on the hands, arm, neck, chest, back, and abdomen and to a lesser extent on the lower extremities. The mucous membranes are spared. The Nikolsky sign is absent. The patient reports recent use of furosemide for periodic leg swelling.