A 17 year-old is brought to see a counselor because her parents are becoming increasingly concerned about her weight loss. She claims that her parents are worrying about nothing, and she is attending the appointment to appease them. She states that she feels fine although her mood is slightly depressed. She denies any problems with sleeping or appetite. She denies substance use. She says that she looks fat but could lose just a few more pounds and she would be just right. She notes that her only problem is that she stopped having her period 3 months ago. She is not sexually active and not pregnant. When questioned separately her parents state that she has been steadily losing weight for the past 8 months. They report that she started dieting when a friend commented that she looked "plump." At the time she weighed approximately 120 pounds. She lost 5 pounds and felt good about people noticing. Since then she has eaten less and less, wears baggy clothes, and dismisses her parents' concerns. She helps plan and prepare elaborate meals when her family entertains. She exercises throughout the day, and her mother reports hearing her do sit ups and jumping jacks.
What is the likely diagnosis? Why did you choose this diagnosis?