Which CK isoenzyme is elevated in muscle diseases?

• A. CK-MM
• B. CK-BB
• C. CK-MB
• D. CK-NN

Answer: (A)

The CK isoenzyme pattern reflects the tissue source of injury, and in skeletal muscle disease the enzyme released into blood comes mainly from muscle fibers. CK has three main forms: CK-MM is predominantly from skeletal muscle, CK-MB from heart, and CK-BB from brain. When muscle is damaged, the serum CK rises with CK-MM as the major elevated isoenzyme, so CK-MM becomes the most prominent CK elevation in muscle disorders. The other isoforms rise primarily with injury to their respective tissues (CK-MB for cardiac injury, CK-BB for brain), so they are not the typical markers for skeletal muscle disease.