A 58-year-old man presents 18 hours after chest pain. Troponin I is elevated. Which isoenzyme pattern of CK would confirm myocardial injury if troponin were unavailable?

• A CK-MM predominance with normal CK-MB
• B CK-BB elevation with normal CK-MM
• C CK-MB fraction <3% of total CK
• D CK-MB fraction >6% of total CK ✓

Explanation

CK-MB constitutes >6% of total CK activity is the diagnostic threshold for myocardial injury; below this level, elevation is more consistent with skeletal muscle origin. CK-MM is the predominant isoform in skeletal muscle; CK-BB is found in brain and rarely elevated in myocardial infarction. The 6% index accounts for the proportional rise in CK-MB relative to total CK.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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