Which biomarker is elevated in muscle injury and has isoforms MM, MB, and BB?

• A. CK
• B. LDH
• C. AST
• D. ALT

Answer: (A)

Creatine kinase is the enzyme released when skeletal muscle is damaged. It exists in three tissue-specific isoforms: CK-MM in skeletal muscle, CK-MB in heart, and CK-BB in brain. When muscle injury occurs, CK leaks into the bloodstream and the total CK rises, with CK-MM being the predominant skeletal muscle isoform. The named isoforms (MM, MB, BB) are characteristic of CK, making it the biomarker most closely associated with muscle injury. Other markers like LDH, AST, and ALT can increase with muscle involvement but do not have this distinct CK isoform pattern, so they are less specific for muscle injury.