The presence of increased CK-MB activity on a CK electrophoresis pattern is most likely found in which condition?

• A. Acute muscular stress following strenuous exercise
• B. Malignant liver disease
• C. Myocardial infarction
• D. Severe head injury

Answer: (C)

CK-MB is a heart-enriched isoenzyme of creatine kinase. When heart muscle is damaged, CK-MB is released into the blood, so its activity increases on a CK electrophoresis pattern, reflecting myocardial injury. This makes it a useful indicator of conditions involving the heart, such as myocardial infarction, where cardiac cell death releases CK-MB early in the course. In contrast, strenuous skeletal muscle effort mainly raises CK-MM, not CK-MB, because the CK-MB fraction is much smaller in skeletal muscle. Severe head injury can cause skeletal muscle breakdown and elevated total CK, but the CK-MB fraction is not specifically increased by brain or head injury. Malignant liver disease does not typically cause a rise in CK-MB. Therefore, an increased CK-MB activity on electrophoresis most strongly points to myocardial infarction.