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
In the treatment of severe bradycardia, all of the following can be the best modality of treatment except 
Select and explain. 
A. Atropine, B. Pacing, C. Isoproterenol, D. Diltiazem
D: Diltiazem
Answer: D: Diltiazem 
Explanation: None