This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Conducting System of Hea, Subject name: Physiology
[{'content': 'Major current for depolarization in Purkinje fiber is of: \nSelect and explain. \nA. Na+, B. Ca++, C. K+, D. Cl-', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Purkinje fiber is a fast-response type or rapidly depolarizing fiber. The rapid depolarization is due a rapid influx of Na+ through the fast Na+ channels. Major current for depolarization in pacemaker cells is due to Ca++', 'role': 'assistant'}]