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MD was born following a normal pregnancy and delivery. His parents were nonconsanguineous, and the family history was unremarkable. He had a 6-year-old brother who was well. MD walked when he was 18 months old, 6 months later than his brother. He was a toe-walker and had large calves. He never ran as well as his brother, and he could not hop on one foot. By 4 years of age, he had difficulty climbing stairs at home and the ladder at the neighborhood playground. His pelvic girdle muscles were weak, he walked with a rocking, side-to-side, waddling gait (Trendelenburg), and he developed lumbar lordosis. He fell more frequently for no apparent reason. His parents became concerned and sought medical advice.

Laboratory testing revealed a serum creatine kinase value 50 times greater than normal. On genetic testing, Duchenne muscular dystrophy (DMD) was...

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