The doctor explains that it's not just the crime that's driving Xingxing crazy. It's also the fact that he's seen a skeleton, which has made him hyper-aware of the horrible events that have taken place in his life. The doctor tells the captain that if they're going to treat him, he should be evaluated for Post Traumatic Stress Disorder, or PTSD. This is a psychological disorder caused by traumatic events, such as a war or a murder, that cause a person to relive those events over and over again. The captain asks if there's anything they can do to help the patient, and the doctor tells him to draw a picture of the skeleton and show it to the captain.