Petter Khant, a 6-year-old boy, is brought to his family doctor by his mother because of concerns that he is not learning in school. He is described as a "high energy child", always on the go. He has a very short attention span. His teacher sent along a note explaining that Petter is well behind the expectations for this age. His classmates are learning the sounds that go with different letters, but Petter does not yet even have a concept of letters or numbers. His vocabulary seems very limited, both receptive and expressive. His mother recalls no concerns about his early development. She remembers him as a generally healthy baby. He started to crawl at 9 months and could walk on his own by 13 months. He only began to use a few single words at 2 years of age. A hearing test done at that time was normal.