<prompt> In a patient with chronic atrial fibrillation with a regular beat of 60min, the most probable cause is :SleepDigitalis toxicitySino nodal blockHypothyroidism  <response>: Answer is B Digitalis toxicity If in the presence of atrial fibrillation, the ventricular rhythm becomes regular and slow 30 to 60m complete hea block is suggested and if ventricular rhythm is regular and rapid 100 min, a tachycardia arising in AV junction or ventricle should be suspected. Digitalis intoxication is a common cause of both phenomenon Digitalis toxicity is a common cause of such a presentation and hence has been picked up as the answer of choice. Infact Non paroxysmal atrial tachycardia with AV block is characteristic of digitalis intoxication. 