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 about cardiac O2 demand is \nSelect and explain. \nA. Directly proportional to mean arterial pressure, B. Inversely proportional to heart rate, C. Inversely proportional to cardiac work, D. Has a constant relation to the external work done by the heart', 'role': 'user'}, {'content': 'Answer: A \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-\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0Rate- Directly proportional\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0Contractility- Directly proportional\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0Intramyocardial tension- Directly proportional\no\xa0\xa0\xa0After load- Directly proportional\no\xa0\xa0\xa0Preload- Directly proportional\no\xa0\xa0\xa0Wall thickness- Inversely proportional\n-\xa0\xa0\xa0\xa0\xa0\xa0\xa0 The oxygen demand of myocardium is not having the same change with varying external works.', 'role': 'assistant'}]