CK Isoenzymes in Myositis — SCE Rheumatology MCQ

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EasyMyositisCK Isoenzymes in MyositisSCE Rheumatology

A 50-year-old woman with active dermatomyositis has symmetric proximal muscle weakness and a serum creatine kinase concentration of 6000 U/L. CK isoenzyme analysis shows that almost all the circulating CK activity is CK-MM. Which tissue is the most likely principal source of the CK elevation?

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Correct answer: B — Skeletal muscle

CK-MM is the principal cytosolic CK isoenzyme in adult skeletal muscle. In this patient, active dermatomyositis, symmetric proximal weakness and a marked CK elevation dominated by CK-MM make skeletal muscle fibre injury the principal source. Brain injury is associated predominantly with CK-BB, while the liver is not a major source of circulating CK-MM. Visceral smooth-muscle injury would not explain this characteristic clinical syndrome. The myocardium contains CK-MM as well as CK-MB, so CK-MM predominance alone does not categorically exclude coexistent cardiac injury. If cardiac involvement is clinically suspected, it requires assessment with appropriate cardiac biomarkers, ECG and imaging rather than CK isoenzyme interpretation alone.

Reference: Kurapati R, Zubair M. Creatine Kinase MB: Diagnostic Utility and Limitations. StatPearls; updated 15 February 2026. https://pubmed.ncbi.nlm.nih.gov/32491523/