This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Pharmacology
[{'content': 'Quinidine exes action on hea by \nSelect and explain. \nA. Ca+ Channel blocker, B. Kt Channel blocker, C. Na+ Channel opener, D. Cl+ Channel opener', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Refer katzung 11e p 237 * Mechanism of action Like all other class I antiarrhythmic agents, quinidine primarily works by blocking the fast inward sodium current (INa). ... The effect of blocking the fast inward sodium current causes the phase 0 depolarization of the cardiac action potential to decrease (decreased Vmax).', 'role': 'assistant'}]