A 22-year-old woman comes to the physician because of

a 2-week history of blue staining of her clothes when she exercises.

She noticed that the stains are most prominent on her underwear and under her

arms. She works as a professional ballet dancer and trains at least 40

hours per week. Physical examination

shows black-blue discoloration of the skin in the axillae, areolae,

and perineum. The remainder of the examination shows no abnormalities. Based on

the location, which type of gland is likely to cause this discoloration?