A 2-month-old infant is brought to clinic for evaluation of a skin lesion present since birth. The parents report it has grown proportionally with the child. Physical examination shows a single, well-demarcated 5-cm hyperpigmented patch on the upper back with multiple dark, coarse terminal hairs arising from within the lesion. The infant is otherwise healthy. Which of the following is the most likely diagnosis, and which statement about prognosis is most accurate?