10 years old Ramu has increasing muscle weakness and raised CPK levels. The most likely defect is in plasma membrane of –
Increase in CPK level with muscle weakness suggests muscle cell damage.
Creatine kinase is an enzyme found mainly in → Skeletal muscle, Heart, Brain
Its levels will be raised in the serum when any of the above mentioned cell is damaged.
There are various isotypes of the enzyme which are specific for difference cells.

                Enzyme                                             Tissue

Creatine kinase (MB isoenzyme)           Heart
Creatine kinase (BB isoenzyme)             Brain
Creatine kinase (MM isoenzyme)         Skeletal muscle
Other enzymes elevated in muscle diseases   → Aldolase, Aminotransferases specially (ACT), Lactate dehydrogenase
Nerves
Muscle fibres
Basement membrane
All body cells