10 years old Ramu was brought by his mother with complaints of increasing muscle weakness. Laboratory findings shows raised CPK levels. The most likely defect is in plasma membrane of:

Significant history of muscle weakness and raised creatinine phosphokinase (CPK) levels are indicative of muscle pathology. Muscular dystrophies are a group of inherited myopathic disorders characterized by progressive muscle weakness and wasting. CPK is the preferred muscle enzyme to measure in the evaluation of myopathies. Damage to muscle causes the CPK to leak from the muscle fiber to the serum. The MM isoenzyme predominates in skeletal muscle, while creatine phosphokinase-myocardial bound (CPK-MB) is the marker for cardiac muscle. Also know: Serum CK can be elevated in normal individuals without provocation, presumably on a genetic basis or after strenuous activity, minor trauma (including the EMG needle), a prolonged muscle cramp, or a generalized seizure. Ref: Amato A.A., Brown, R.H. (2012). Chapter 387. Muscular Dystrophies and Other Muscle Diseases. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.