Soru

A 25-year-old nulligravid woman presents to the clinic reporting an inability to conceive for the past 16 months. She and her partner have regular unprotected intercourse. The patient has a history of unpredictable menstrual cycles since menarche at age 12, often going 3 to 4 months between menses. Over the past few years, she has noted progressive development of dark, coarse hair on her upper lip, chin, and chest. She takes a daily multivitamin. Her BMI is $31 \text{ kg/m}^2$. Physical examination is notable for hirsutism, moderate facial acne, and thickened, velvety, hyperpigmented skin on the nape of her neck. Pelvic examination is normal. Transvaginal ultrasonography reveals bilaterally enlarged ovaries, each containing about 20 small peripheral follicles. A urine pregnancy test is negative. Serum thyroid-stimulating hormone and prolactin levels are within normal limits.

To formally confirm the most likely diagnosis, which of the following diagnostic tests is required to exclude a phenotypically similar mimicking condition?