A 4-year-old boy is brought to the physician by his parents because he tires easily. Physical examination reveals weakness in the pelvic and shoulder girdles and enlargement of the child's calf muscle. Serum levels of creatine kinase are elevated. A biopsy of calf muscle shows marked variation in size and shape of muscle fibers. There are foci of muscle fiber necrosis, with myophagocytosis, regenerating fibers, and fibrosis. Molecular diagnostic assays would most likely show alterations in the length of the primary transcript for which of the following muscle-associated proteins?

Duchenne muscular dystrophy (DMD) is a severe, X-linked condition characterized by progressive degeneration of muscles, particularly those of the pelvic and shoulder girdles. A milder form of the disease is known as Becker muscular dystrophy (BMD). Both DMD and BMD are caused by a deficiency of dystrophin, a member of the family of membrane cytoskeletal proteins, which includes a-actinin and spectrin. The protein is located on the cytoplasmic face of the plasma membrane of muscle cells and is linked to it by integral membrane glycoproteins (dystrophin-associated glycoprotein complex), which in turn, are bound to extracellular laminin. It has been proposed that the absence of dystrophin leads to a defective membrane that is damaged during contraction, an effect that predisposes to death of the myocyte. Serum levels of creatine kinase are increased. Glycogen phosphorylase gene mutations (choice D) are found in patients with McArdle disease.Diagnosis: Duchenne muscular dystrophy