You are seeing a 14-year-old female in a primary care setting who has presented with her mother. During the history, the mother informs you that her daughter has lost a significant amount of weight recently and is counting her calories at mealtimes. Her mother is concerned about her. When you question the patient, she informs you that she has an intense fear of gaining weight and does not see her current low body weight as an issue. What is the most likely diagnosis?