A 22-year-old female is coming to you after witnessing her f…

A 22-year-old female is coming to you after witnessing her friend Sally die in a motor vehicle accident. She was also a passenger in the vehicle. The accident occurred five weeks ago and since then she has been experiencing flashbacks of the accident and nightmares that are waking her from sleep. She feels guilty that she lived through the accident and Sally did not make it. She does not want to talk about the event and has been avoiding any of her old friends of whom she shared with Sally. She cannot ride in a car without extreme distress. She cannot concentrate at school and is considering withdrawing from her classes and moving back home with her parents. Which of the following is the most likely diagnosis?