A 58-year-old man presents with crushing chest pain for 3 hours. Troponin I is markedly elevated. Which isoform of creatine kinase (CK) would be characteristically elevated in this patient, and what is its subunit composition?
- A CK-MM (two M subunits) — predominantly from skeletal muscle
- B CK-MB (one M + one B subunit) — predominantly from myocardium ✓
- C CK-BB (two B subunits) — predominantly from brain
- D CK-MB2 (mitochondrial isoform) — released only in irreversible injury
Correct answer: B. CK-MB (one M + one B subunit) — predominantly from myocardium
Explanation
CK-MB, composed of one M and one B subunit, is the predominant isoform in cardiac muscle and rises within 4–6 hours of myocardial infarction, peaking at 12–24 hours. CK-MM predominates in skeletal muscle and CK-BB in brain; CK-MB >6% of total CK is diagnostically significant for MI. While troponin is now the preferred biomarker, CK-MB remains clinically relevant for timing of reinfarction.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.