To aid in diagnosing skeletal muscle disease, which serum enzyme measurement is most useful?

• A. Creatine kinase
• B. Alkaline phosphatase
• C. Aspartate aminotransferase
• D. Alanine aminotransferase

Answer: (A)

The main idea is that markers released from damaged tissue reflect the tissue involved. For skeletal muscle disease, a muscle-dominant enzyme provides the most sensitive and specific signal. Creatine kinase is abundant in skeletal muscle, and when muscle fibers are injured, CK leaks into the bloodstream in large amounts. This makes serum CK a highly sensitive indicator of muscle damage and disease activity, more so than other enzymes that are not specific to muscle.

Alkaline phosphatase mainly reflects bone and liver activity, while AST and ALT are liver-associated enzymes (though AST is also present in muscle, it is less specific and often influenced by liver disease). Because CK elevation directly correlates with muscle injury, it’s the best choice for diagnosing and monitoring skeletal muscle disorders.