A 22-year-old male presents 2 weeks after a motor vehicle accident in which he was a passenger and his friend was killed. He reports intrusive memories of the accident, recurrent nightmares, and flashbacks triggered by traffic sounds. He avoids riding in cars and refuses to drive past the accident location. He describes feeling detached from others and an inability to experience positive emotions. He reports a persistent inability to remember key details of the accident. He has an exaggerated startle response, difficulty sleeping, and hypervigilance when near roads. Symptoms began 3 days after the event. What is the MOST likely diagnosis?