Two weeks ago, Leo, age 25, was hunting with his best friend when his friend tripped over a root and accidentally shot himself to death. He doesn't remember exactly how he got himself and his friend's body out of the woods. Every night he has nightmares about not being able to save him, making it almost impossible to sleep. He can't concentrate or stay focused during conversations. Most of the time, he says he feels numb. But when asked to talk about what happened, he gets very upset. Leo may suffer from ___.