A 20-year-old man has exhibited slowly progressive muscle weakness over the past 5 years. He has no myalgias. He now has difficulty ambulating and picking up heavy objects, even though he used to be a weightlifter. On physical examination there is 4/5 motor strength in proximal muscle groups. Laboratory studies show a serum creatine kinase of 1450 U/L. A muscle biopsy immunohistochemical staining shows focal positivity for dystrophin around the fibers. Which of the following is the most likely diagnosis?