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': 'Phase 1 of Myocardium AP is mainly due to: \nSelect and explain. \nA. K+ efflux, B. Na+ channel blockage, C. Ca2+ efflux, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Phase\xa00\xa0(depolarization),\xa0fast\xa0sodium\xa0channels\xa0open.\nPhase 1 (initial repolarization), fast sodium channels close.\xa0\nPhase\xa02\xa0(plateau),\xa0calcium\xa0channels\xa0open\xa0and\xa0fast potassium\xa0channels\xa0close.\nPhase 3 (rapid repolarization), calcium channels close and slow potassium channels open.\nPhase 4 (resting membrane potential) averages about −90 millivolts.\nKey Concept:\n\nPhase 1 of myocardium AP is due to K+ opening >> inactivation of Na+ channel.\xa0\xa0\nPhase 2 of myocardium AP starts with Ca++ entry.\nLong duration of myocardium AP is due to phase 2.\n\n\u200b', 'role': 'assistant'}]