When he was about 12, a now 41-year-old man noticed that the skin on his left chest was darkening. For several years afterward, the darkness spread and deepened, and the area became hairy. In young adulthood, he experienced occasional outbreaks of what looked like acne on the lesion; this eventually cleared.
He now finds the hairiness increasingly bothersome, so he shaves the worst parts of it. Upon consulting his primary care provider, he was assured that the lesion is “a birthmark.” Unsatisfied with this answer, the patient took the advice of a friend and decided to consult dermatology.
A polygonal, hyperpigmented, hypertrichotic patch covers most of the patient’s left pectoral area. The lateral margin is irregular but well-defined. There is obvious partial regrowth of the shaved hair on the lateral margin, but it stops abruptly at that point.
The breast and surrounding tissue appear normal. No areas of hyperpigmentation or hypertrichosis are seen elsewhere.
What is the diagnosis?