Problem: A 16-year-old high school student is brought into the physician's office by her mother, who is worried because her daughter insists on dieting because she thinks she is too fat. The daughter is 5ft 6in tall and weighs 96lb. Her history reveals that she is a straight-A student, plays in the orchestra, and is on the track team. Although she had been having regular menstrual periods, she has not had a period in 4 months. She is given a tentative diagnosis of anorexia nervosa. What is the physiologic reason for her amenorrhea?