In a patient with chronic atrial fibrillation with a regular beat of 60/min, the most probable cause is :

Answer is B (Digitalis toxicity) "If in the presence of atrial fibrillation, the ventricular rhythm becomes regular and slow (30 to 60/m) 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."