Review Question - QID 215092

A 5-year-old boy presents to his pediatrician for behavioral issues. The child is struggling to perform in school and has not made any friends. He often plays by himself and avoids others. He recently pushed another child who tried to give him a hug while he was crying. The parents adopted the patient 1 year ago when he was taken from his mother as she was addicted to cocaine and had neglected him. Since being adopted, his parents have noticed that the child prefers to play alone in his room. He does not interact much with them, does not like to be hugged or touched, and usually seems sad. He has a strict routine of going to his room and playing with his toys in his closet after dinner. His parents tried to make a rule that he has to spend time outside of his room after dinner, but the child reacted with rage when this was enforced. He has met normal developmental milestones, has no significant medical history, and takes no medications. His temperature is 98.6°F (37.0°C), blood pressure is 105/65 mmHg, pulse is 100/min, and respirations are 25/min. Exam reveals a withdrawn child who avoids eye contact with the pediatrician. He answers questions with short phrases. His neurological exam is unremarkable. Which of the following is the most likely diagnosis?