A 9-year-old child has been refusing to go to school for the past two months. The child's teacher reports that the child would continually cry after being dropped off at school, causing classmates to tease, leading to the child becoming further isolated. Previously the child had seen the pediatrician for repeated headaches. The parents decided to obtain psychiatric evaluation after the child began to have frequent nightmares. Which of the following is the most likely diagnosis?

A. Social phobia
B. Specific phobia
C. Somatoform disorder
D. Separation anxiety disorder
E. Posttraumatic stress disorder (PTSD)