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: Physiology
[{'content': 'True regarding myocardial O2 demand? \nSelect and explain. \nA. Inversely related to heart rate, B. Has constant relation to external cardiac work, C. Directly proportional to duration of systole, D. Is negligible at rest', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Oxygen demand depends on the work being done. The work done is to pump the blood which is in the ventricles (preload) against the resistance in the aorta (afterload).\n\xa0So, myocardial oxygen demand depends on\n\nRate - Directly proportional\nContractility - Directly proportional\nIntramyocardial tension - Directly proportional\nAfter load - Directly proportional\nPreload - Directly proportional\nWall thickness - Inversely proportional\n\nThe oxygen demand of myocardium is not having the same change with varying external works.', 'role': 'assistant'}]