Question

A 26-year-old woman is evaluated in the office alongside her husband due to primary infertility. The couple has engaged in regular, unprotected vaginal intercourse for the past 16 months without achieving pregnancy. The patient reports that her menstrual periods are highly unpredictable, typically occurring at 60- to 90-day intervals. Over the last three years, she has noticed the gradual onset of coarse, dark hair growing on her upper lip and chin, which she manages by daily plucking. She has no chronic medical conditions and takes no medications other than a prenatal vitamin. Vital signs are within normal limits. Her body mass index is $29 \text{ kg/m}^2$. Physical examination demonstrates a female escutcheon and normal external genitalia without clitoromegaly. A bimanual pelvic examination is unremarkable. The partner's semen analysis parameters are within normal reference ranges. A transvaginal ultrasound reveals bilaterally enlarged ovaries, each containing 15 to 20 small, peripherally oriented follicles. Serum concentrations of prolactin and thyroid-stimulating hormone are normal. The clinician suspects polycystic ovary syndrome (PCOS). To definitively establish this diagnosis, which of the following is the most appropriate next step in the laboratory evaluation?