Review Question - QID 214976

A 17-year-old girl presents to the clinic reporting 7 months without a menstrual period. Menarche was at age 12 and she had regular periods up until 2 years ago. At that time, her periods became less regular until around 7 months ago when they ceased altogether. She is otherwise healthy, participates in multiple school sports teams, and has been training rigorously for upcoming competitions. She denies alcohol use, smoking, and recreational drugs. She is not sexually active and does not take oral contraceptives. Her temperature is 36.7°C (98°F), blood pressure is 121/80 mmHg, pulse is 62/min, respirations are 11/min, oxygen saturation is 100% on room air, and BMI is 20 kg/m^2. Her pelvic exam reveals an anteverted uterus, no adnexal masses, a normal-appearing cervix with no cervical motion tenderness, and normal vaginal anatomy. Which of the following is the most likely cause of this patient's amenorrhea?