A 32-year-old woman with a history of regular 28-day cycles presents with 2 months of amenorrhea. She is not sexually active and denies pregnancy symptoms. Urine β-hCG is negative. TSH, prolactin, and FSH are all normal. Pelvic ultrasound shows normal endometrial thickness (8 mm) and normal ovaries. A progestin challenge test (medroxyprogesterone acetate 10 mg daily for 5 days) is performed, and she does not menstruate within 7 days. What is the most appropriate next step?