A 5-year-old boy with history of childhood speech and motor delay is referred to the neurology clinic for “abnormal gait.” His mother reports he has always been “clumsy” and a slower runner than other kids his age. However, over the last 3 years he has been slowly worsening, and he has recently started falling and having difficulty walking up the stairs or rising from a chair. Over the past year his parents have noticed he has difficulty lifting things over his head. He says sometimes his calves hurt after walking.