This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
In a patient with chronic atrial fibrillation with a regular beat of 60/min, the most probable cause is : 
Select and explain. 
A. Sleep, B. Digitalis toxicity, C. Sino nodal block, D. Hypothyroidism
B: Digitalis toxicity
Answer: B: Digitalis toxicity 
Explanation: 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."