Diagnostic Tests and Procedures Practice Test 2026 - Free Diagnostic Practice Questions and Study Guide

Muscle injury releases intracellular enzymes into the bloodstream, and one enzyme that is highly concentrated in both skeletal and cardiac muscle is creatine kinase. In a heart attack, the CK-MB isoenzyme rises as cardiac muscle is damaged, while in muscular dystrophy ongoing muscle membrane breakdown leads to markedly elevated CK levels overall. This combination—sensitivity to muscle damage and the presence of muscle-specific isoforms—makes creatine kinase the best indicator among the options for both heart and muscle injury.

The other enzymes aren’t as specific. Alkaline phosphatase mainly reflects bone and liver activity; aspartate aminotransferase is found in liver and muscle but isn’t as muscle-focused; lactate dehydrogenase is widespread and rises in many conditions, reducing its usefulness as a muscle-specific marker.