Physical examination of a 14 year old boy demonstrates six coffee-colored skin macules up to 3 cm in diameter. No other skin lesions are noted, but a small mass lesion is felt in the subcutaneous tissues below two of the macules. These masses are most likely closely associated with which of the following structures?

The lesions are cafe au lait spots. Isolated cafe au lait spots are common in the general population, but more than 3 or 4 cafe au lait spots suggests the possibility of neurofibromatosis. Apparently, the peripheral nerve tumors (neurofibromas) that are pa of this syndrome sometimes induce hyperpigmentation of the overlying skin, producing the cafe au lait spots. The arrector pili are the small muscles of the hair follicle that give rise the sensation of "goose-bumps"; it is thought that some leiomyomas of the skin may arise from these tiny muscles. Hair follicles, sebaceous glands, and sweat glands can give rise to a variety of benign and, very rarely, malignant tumors of the skin that do not usually involve subcutaneous tissues. Ref: Landthaler M., Baumler W., Hohenleutner U. (2012). Chapter 239. Lasers and Flashlamps in Dermatology. In L.A. Goldsmith, S.I. Katz, B.A. Gilchrest, A.S. Paller, D.J. Leffell, N.A. Dallas (Eds), Fitzpatrick's Dermatology in General Medicine, 8e.