A 28-year-old woman presents to the outpatient clinic complaining of frequent nightmares, intrusive flashbacks of a motor vehicle collision, avoidance of driving or discussions about the accident, irritability, hypervigilance, and difficulty sleeping. She was involved in the collision 6 weeks ago and sustained minor injuries. Since then, she has had these distressing symptoms nearly every day. She reports that the symptoms began about two weeks after the accident and have not improved. Physical examination is unremarkable aside from mild tachycardia. Which of the following is the most likely diagnosis?