A 19-year-old woman is evaluated for amenorrhea and fatigue. Her body mass index is 15.5 kg/m2. She reports an intense fear of gaining weight and perceives herself as overweight despite being visibly underweight. Over the past 4 months she has also had recurrent episodes of eating large amounts of food followed by self-induced vomiting and laxative misuse. On examination she has bradycardia, cool extremities, and fine downy body hair. Which of the following is the most likely diagnosis?