Which enzyme is more closely associated with muscle tissue than with liver tissue?

• A. Alanine aminotransferase
• B. Creatine kinase
• C. Alkaline phosphatase
• D. Gamma-glutamyltransferase

Answer: (B)

Creatine kinase is the enzyme most closely linked to muscle tissue because it sits in high abundance in skeletal and cardiac muscle and plays a key role in rapidly regenerating ATP during muscle contraction by transferring a phosphate from phosphocreatine to ADP. When muscle cells are damaged, CK leaks into the blood, making its serum levels a classic marker of muscle injury. The other enzymes are less specific to muscle: alanine aminotransferase is concentrated mainly in the liver (with some presence in muscle), alkaline phosphatase is abundant in bone and bile ducts, and gamma-glutamyltransferase is a liver/biliary marker. So the muscle-specific enzyme among these is creatine kinase.