This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Myocardial Infraction, Subject name: Pathology
Why is Troponin C not used for MI diagnosis? 
Select and explain. 
A. Ttroponin C binds to calcium, B. Troponin C is present in both cardiac and smooth muscle, C. Troponin C is present in both cardiac and skeletal muscle, D. It elevates late after onset of MI.
B: Troponin C is present in both cardiac and smooth muscle
Answer: B: Troponin C is present in both cardiac and smooth muscle 
Explanation: Ans. (b) troponin C is present in both cardiac and smooth muscleRef: Harrison 19th ed. /265e-7* Troponins of cardiac origin are regulatory proteins that control the calcium mediated interaction of Actin and Myosin.* The troponin complex consists of three subunits: Troponin T, which binds to Tropomyosin and facilitates contractions; Troponin I, which binds to Actin and inhibits Actin/Myosin interaction; and Troponin C, which binds the calcium ions* The amino acid sequence of the skeletal and cardiac isoforms of Troponin T and Troponin I are sufficiently dissimilar and therefore detectable by monoclonal antibody assay.* Troponin C is not used clinically because both cardiac and smooth muscle share Troponin C isoforms