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
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 ? 
Select and explain. 
A. Digitalis toxicity, B. Incomplete heart block, C. Stroke, D. Subarachnoid Haemorrhage
A: Digitalis toxicity
Answer: A: Digitalis toxicity 
Explanation: None