ms. gibbs has chosen a military career. a few months ago, she served as part of a supply line for a combat zone. her unit came under enemy fire on two occasions. the second time, one of her close friends was killed and two were injured. ms. gibbs is having difficulty sleeping, and at times she feels anxious and afraid. she has always been a strong, courageous person, and she is very troubled by these feelings. she wonders if she is going crazy. what is the most likely diagnosis?