The lesion on this 12-year-old girl’s trunk has been present since birth, growing slowly as she has. Recently, an abrasion sustained during a basketball game caused the lesion to become swollen and tender. It has since returned to its original size and nontender state, but the change in appearance raised enough concern to prompt dermatologic consultation.
The child is otherwise healthy and reports having had very few sunburns, tanning easily (though seldom).
The lesion—an oval, nevoid, hair-bearing, uniformly brown plaque with a mammilated surface—is located on the right lower anterior abdominal wall and measures just short of 3 cm x 2 cm. It bears no sign of the recent trauma. The margins are clearly defined, and the lesion is nontender on palpation. No increased warmth is detected.
Overall, the patient’s type III skin has little, if any, evidence of excessive sun exposure.
What is the diagnosis?