David Beatty is a 5 year old boy brought to his family doctor by his mother. She is concerned about his clumsiness. He had always been a little 'slow' with his gross-motor milestones. His mother recalled that he did not start walking until he was 20 months old. As he has gotten older, he has seemed less able to keep up with other children his age. As an example she explains that she sees other kindergarten children at the school playground who all seem to be able to run, climb and hop without difficulty. But not David. He is slow, cannot climb well, and falls very often. He also tires out quickly. She worries that maybe he has a problem with the bones in his legs, or maybe a problem with his hips. On examination, David is 111 cm tall (50th percentile), weighs 21 kg (75th percentile), and has a head circumference of 52 cm (75th percentile). He is a happy boy and is eager to tell stories about his teacher and classmates. When standing he has a prominent lordotic curve to his back. He has difficulty climbing up on the examination table, but insists on doing it himself. His heart, lungs and abdomen appear normal. His joints have full range of motion and he does not appear to have any joint or bone pain. His muscle bulk appears to be good - in fact he has large, muscular-looking calves. His deep tendon reflexes are normal (grade 2 out of 4).