A 23-year-old female college student presents with amenorrhea for 10 months. She had menarche at age 14 and normal regular menses every 28 days until this year. She is in good health and not taking any medications. She is 5 feet 4 inches tall and weighs 130 pounds. Her examination, including a pelvic exam, is normal. Beta-hCG is < 5 mIU/mL, and TSH is 5.0 mU/L (normal 0.35-6.7 mU/L). What is the most appropriate next diagnostic test to help determine the cause of amenorrhea in this patient?
1) Serum 17-hydroxyprogesterone level
2) Serum prolactin level
3) Pelvic ultrasound
4) Serum LH and FSH levels
5) Brain MRI