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
[{'content': 'In a patient with chronic atrial fibrillation with a regular beat of 60/min, the most probable cause is : \nSelect and explain. \nA. Sleep, B. Digitalis toxicity, C. Sino nodal block, D. Hypothyroidism', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 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."', 'role': 'assistant'}]