Which tissue contains CPK-MB?

• A. Skeletal muscle
• B. Cardiac muscle
• C. Brain
• D. Liver

Answer: (B)

CK-MB is the creatine kinase isoenzyme that is predominantly found in heart muscle. When cardiac cells are damaged, CK-MB is released into the bloodstream, which is why elevated levels serve as a marker of myocardial injury. Although small amounts can be present in skeletal muscle, the heart contains this isoenzyme most abundantly, making cardiac muscle the tissue most associated with CK-MB. For context, CK-BB is the brain form and CK-MM is the skeletal muscle form, so the presence of CK-MB in the blood most strongly points to myocardial rather than brain or skeletal muscle sources. Clinically, CK-MB rises a few hours after an infarct and returns to baseline within a couple of days, though troponins are now favored for specificity.