A 23-year-old computer programmer comes to the office for an annual examination. She has recently become sexually active and wants to start oral contraception. Her only complaint is that the skin in her armpits has become darker. She states it looks like dirt; she scrubs her skin nightly with soap and water, but the colour stays. Her past medical symptoms consist of acne and mild obesity. Her periods have been irregular for 3 years. Her mother has type 2 diabetes and her father has high blood pressure. The client denies tobacco but drinks four to five drinks on Friday and Saturday nights. She denies any illegal drug use. Examination shows a mildly obese woman breathing comfortably. Her vital signs are unremarkable. Inspection of the axillae reveals dark velvet-like skin. Her annual examination is otherwise unremarkable. What disorder of the breast or axilla is she most likely to have?