A 16-year-old male is referred by his pediatrician for evaluation of significant weight loss and nutritional deficiency. His BMI has dropped from the 45th percentile to the 8th percentile over 14 months. He reports that he has always been a 'picky eater' but his food repertoire has narrowed dramatically after a choking incident on a piece of steak 18 months ago. He now eats only five foods: plain white rice, plain pasta, crackers, applesauce, and vanilla yogurt. He avoids all foods with complex textures, stating they make him feel like he will choke. He denies any desire to lose weight, expresses no distortion in body image, and states he wishes he could eat more foods. Laboratory results reveal iron-deficiency anemia and vitamin D deficiency. He reports feeling embarrassed at school because he cannot eat in the cafeteria and has started declining social invitations involving meals. Which of the following diagnoses best explains this presentation?