A24-year-old nulliparous woman comes into the office because she has not had her menses for six months. she is in good health and not taking any medications. she is not sexually active. she does well in graduate school, despite her demanding new program. her height is 5 feet 6 inches and her weight is 104 pounds. her vital signs are normal. her physical examination, including a pelvic examination, is completely normal. what is the most likely reason for her amenorrhea?