<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 40-year-old man comes to the physician because of a 1-week history of palpitations and dizziness. His pulse is 65min and regular. An ECG is shown. A drug is administered that inhibits non-dihydropyridine calcium channels in the heart and his symptoms improve. The drug administered to the patient most likely has which of the following effects on the cardiac conduction system?? A: Prolongation of Purkinje fiber refractory period, B: Decrease in bundle of His refractory period, C: Delay in ventricular myocardial repolarization, D: Decrease in ventricular myocardial action potential duration, E: Prolongation of AV node repolarization,  <response>: This is a question about the effects of a drug on the cardiac conduction system. The patient has palpitations and dizziness, with a regular pulse. The ECG shows atrial fibrillation with a rapid ventricular response. A drug that inhibits non-dihydropyridine calcium channels is given, which improves the patients symptoms. Based on the information provided, this drug likely acts on the AV node to decrease conduction through the AV node from the atria to the ventricles. This would explain the improvement in symptoms. The drug is not acting directly on ventricular myocardial repolarization or refractory periods, so answers C and D can be eliminated. Answer A about Purkinje fibers is unlikely since these are not typically involved in AV conduction. Answer E about prolonging AV node repolarization is the most likely effect of this drug, which would slow AV conduction through the node and improve the patients rapid ventricular response to atrial fibrillation.E: Prolongation of AV node repolarization 