A 37-Year-Old Man with No Significant Medical History Comes to the Office

A 37-year-old man with no significant medical history comes to the office due to intermittent palpitations.  He has a healthy lifestyle, exercises daily, and does not use tobacco, alcohol, or illicit drugs.  ECG shows atrial fibrillation with rapid ventricular response.  Echocardiogram reveals normal heart function with no significant valvular disease.  The patient is started on an antiarrhythmic medication that is known to be effective against both atrial and ventricular arrhythmias.  On ECG, this medication causes QRS prolongation with only minimal increase in QT interval duration.  Which of the following regions of the action potential curve is most affected by this drug?

A) A
B) B
C) C
D) D
E) E

Correct Answer:

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