A 17-year-old girl has irregular menstrual periods for 1 year. Periods occur every 3 to 4 months and last 15 to 20 days at a time. She reached menarche at age 12 and initially had irregular periods. She also reports that she developed hirsutism this year and gained about 7 kg, changes that she attributes to an increase in her intake of fast food. Physical examination demonstrates facial hair along the upper lip, chin, neck, and jawline as well as hair on the abdomen. An external genitourinary examination is unremarkable. Thyroid tests, a prolactin level, and a 17-hydroxyprogesterone level are all normal. Which test is most likely to confirm the diagnosis in this patient?
