This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Anatomy
All of the following statements are true about quinidine except: 
Select and explain. 
A. It blocks myocardial Na+ channels primarily in the open state, B. It has no effect on myocardial K+ channels, C. It produces frequency dependent blockade of myocardial Na+ channels, D. It delays recovery of myocardial Na+ channels
B: It has no effect on myocardial K+ channels
Answer: B: It has no effect on myocardial K+ channels 
Explanation: (Ref: KDT 6/e p511, 512) Quinidine is a class I antiarrhythmic agent. It blocks Na+ channels in the open state and also delays the recovery of these channels. Blockade by these antiarrhythmic agents is frequency dependent. Class Ia antiarrhythmics, in addition blocks myocardial K+ channels also.