A 34-year-old male firefighter presents 3 weeks after being trapped under a collapsed beam during a structure fire. He was briefly unconscious and witnessed a colleague sustain fatal injuries. Since the event, he has experienced intrusive flashbacks of the collapse, recurrent nightmares, severe hypervigilance, an exaggerated startle response, emotional numbing, avoidance of fire stations and news reports about fires, difficulty sleeping, and irritability. He reports feeling detached from his family and states the world feels 'unreal.' He has been unable to return to work. Symptom onset was within 48 hours of the event. Which is the most accurate diagnosis at this time?