Afather brings his 14-year-old son to your clinic because his teacher has concerns about his poor school performance and maladjusted behavior. he has poor grades in all subjects, is extremely shy, and has always had difficulty in adjusting socially. on examination, he is at the 95th percentile for height and 5th percentile for weight. it is very difficult to engage him in conversation. the testes are prepubertal, he has mild hypospadias, and he has no secondary sexual characteristics. which of the following is the most likely cause of his pubertal delay?