This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Irreversible injury in myocardium occurs at ? 
Select and explain. 
A. 2 minutes, B. 30 minutes, C. 2 hours, D. 5 hours
B: 30 minutes
Answer: B: 30 minutes 
Explanation: Ans. is 'b' i.e., 30 minutes The metabolic rate of the hea is high, and its stores of substrate are low. So, hea is critically dependent on a continuous supply of oxygen and nutrients. Ischemia of myocardium induces profound functional, biochemical and morphological consequences. Myocardial function is more sensitive to isc_hemia (loss of contractility occurs within 60 seconds) than myocardial structure (irreversible injury occurs in 20-40 minutes, thus myocardial necrosis begins at approximately 30 minutes after coronary occlusion. Key events in ischemic cardiac myocytes Feature Time Onset of ATP depletion Seconds Loss of contractility < 2 minutes ATP reduced to 50% 10 minutes ATP reduced to 10% 40 minutes Irrversible injury 2040 min Microvascular injury > 1 hr