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': 'Inward flow of Na+  in heart leads to: \nSelect and explain. \nA. Plateau phase, B. Action potential, C. Repolarization, D. No change', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Phase 0 (depolarization), fast sodium channels open. When the cardiac cell is stimulated and depolarizes, the membrane potential becomes more positive. Voltage-gated sodium channels (fast sodium channels) open and permit sodium to rapidly flow into the cell and depolarize it. The membrane potential reaches about +20 millivolts before the sodium channels close.\nKey Concept:\nInﬂux of Na+ via fast Na channel leads of phase 0 or beginning of AP in myocardium.\nReference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no111', 'role': 'assistant'}]