This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CHF, Angina Pectoris and Myocardial Infarction, Subject name: Pharmacology
Mechanism of action of digitalis in atrial fibrillation is: 
Select and explain. 
A. By decreasing cardiac contractility, B. Na+ K+ ATPase inhibition, C. Increase in refractoriness of AV nodal tissue, D. By causing bradycardia
C: Increase in refractoriness of AV nodal tissue
Answer: C: Increase in refractoriness of AV nodal tissue 
Explanation: Digoxin reduces ventricular rate in A. Fib. by decreasing the number of impulses that are able to pass down the A-V node and bundle of His. It increases ERP of A-V node by direct, vagomimetic and antiadrenergic actions --> Minimum interval between consecutive impulses that can successfully traverse the conducting tissue is prolonged.