A boy has developed pearly, umbilicated, firm papules on his thighs. What is the cause?
Over a six-month period, a 9-year-old boy developed on his thighs and pubic area a progressive number of 2 to 4 mm diameter pearly papules, some of which had a small central dimple. The papules were focally aggregated, and one lesion was obscured by an inflammatory reaction. Skin biopsy of the papule revealed an epithelium-lined canal containing numerous eosinophilic bodies that appeared to develop within the surrounding keratinocyte lining.