A 4-year-old boy is discovered to have an IQ of 60 and is referred to a pediatrician. His mother reports that he was born 5 weeks premature by spontaneous vaginal delivery after an uncomplicated pregnancy. His developmental history is notable for initially sitting at 9 months, saying his first word at 16 months, walking at 19 months, and climbing up stairs at 3.5 years. He plays cooperatively with the other children. On physical examination, he is cooperative with the examiner, responding in three-word sentences. He walks on his tiptoes with a scissored gait. His legs are hypertonic bilaterally with brisk patellar reflexes; he has ankle clonus and upgoing toes bilaterally. What is the most likely diagnosis?
(A) Autism
(B) Cerebral palsy
(C) Down’s syndrome
(D) Duchenne’s muscular dystrophy
(E) Fragile X syndrome
(A) Autism
(B) Cerebral palsy
(C) Down’s syndrome
(D) Duchenne’s muscular dystrophy
(E) Fragile X syndrome