This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Medicine
[{'content': 'A person with mitral regurgitation and atrial fibrillation presents with syncope. On examination the person has a hea rate of 55. What is the most probable cause ? \nSelect and explain. \nA. Digitalis toxicity, B. Incomplete hea block, C. Stroke, D. Subarachnoid Haemorrhage', 'role': 'user'}, {'content': 'Answer: A \nExplanation: ref Harrison 20th Ed pg 1456', 'role': 'assistant'}]