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 its action on the hea by \nSelect and explain. \nA. Ca2+ channel blockade, B. Na+ channel blockade, C. K+ channel opening, D. Cl- channel opening', 'role': 'user'}, {'content': 'Answer: B \nExplanation: It is d-isomer quinidine is used as an antiarrhythmic drug. Quinidine blocks myocardial Na channels. Class 1a group of drugs- quinidine, procainamide, disopyramide Ib: Lidocaine, phenytoin Ic-encainide, flecainide Ref: KD tripathi 8th ed.', 'role': 'assistant'}]