A 4 year old boy's parents complain that their child has difficulty walking. The child rolled, sat and first stood at normal ages and first walked at 13 months of age. Over the past several months however, the family has noticed an increase inward curvature of the lower spine as he walks and that his gait has become more waddling. On examination, the child is oriented and interactive with the physician with an average body built. Enlargement of his calves has been noticed