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
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 ? 
Select and explain. 
A. Digitalis toxicity, B. Incomplete hea block, C. Stroke, D. Subarachnoid Haemorrhage
A: Digitalis toxicity
Answer: A: Digitalis toxicity 
Explanation: ref Harrison 20th Ed pg 1456