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': 'A person with mitral regurgitation and atrial fibrillation presents with syncope. On examination the person has a heart rate of 55. What is the most probable cause ? \nSelect and explain. \nA. Digitalis toxicity, B. Incomplete heart block, C. Stroke, D. Subarachnoid Haemorrhage', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]