Assertion (A) is true because cardiac troponins (T and I) are sensitive and specific markers for myocardial infarction.
They start to rise within 2 to 4 hours after cardiac muscle injury.
Reason (R) is also true because during an MI, damage to myocardial cells causes leakage of cellular contents, including enzymes and proteins like troponins, into the bloodstream.
Thus, the reason correctly explains why troponins become detectable after MI.
Therefore, both statements are true and (R) correctly explains (A).