A 3-year-old boy's parents complain that their child has difficulty walking. The child rolled, sat, and first stood at essentially normal ages and first walked at 13 months of age. Over the past several months, however, the family has noticed an increased inward curvature of the lower spine as he walks and that his gait has become more "waddling" in nature. On examination, you confirm these findings and also notice that he has enlargement of his calves. Which of the following is the most likely diagnosis?

The most common form of muscular dystrophy is Duchenne muscular dystrophy. It is inherited as an X-linked recessive trait. Male infants are rarely diagnosed at birth or early infancy since they often reach gross milestones at the expected age. Soon after beginning to walk, however, the features of this disease become more evident. While these children walk at the appropriate age, the hip girdle weakness is seen by the age of 2 years. Increased lordosis, while standing, is evidence of gluteal weakness. Gower sign (use of the hands to "climb up" the legs in order to assume the upright position) is seen by 3 to 5 years of age, as is the hip waddle gait. Ambulation ability remains through about 7 to 12 years of age, after which use of a wheelchair is common. Associated features include mental impairment and cardiomyopathy. Death caused by respiratory failure, heart failure, pneumonia, or aspiration is common by early adulthood.