A 35-year-old woman presents to your clinic complaining of recurrent nightmares and “reliving” a traumatic event. Ten weeks ago, she was the driver in a motor vehicle collision in which a pedestrian was killed. Since then, she reports nightly dreams of the crash, intrusive memories triggered by sounds of brakes, avoidance of driving, irritability, difficulty concentrating, hypervigilance, and exaggerated startle response. These symptoms have persisted despite counseling and over-the-counter sleep aids. What is the most likely diagnosis?