This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
10 years old Ramu has increasing muscle weakness and raised CPK levels. The most likely defect is in plasma membrane of – 
Select and explain. 
A. Nerves, B. Muscle fibres, C. Basement membrane, D. All body cells
B: Muscle fibres
Answer: B: Muscle fibres 
Explanation: 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