The anxious parents of a previously healthy 19-month-old boy bring the child to the emergency department for evaluation of progressive rash that began 4 months ago. The skin eruption began as small blisters on his knees, which became tense and ruptured, eventually evolving to red-pink scaly plaques. Over the next few months, the boy developed similar lesions on his hands, elbows, neck, perineal area, and face, with sparing of the mucous membranes.
