This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Pharmacology
Calcium channel blocker with maximum effect on conduction in hea is: 
Select and explain. 
A. Phenylamine, B. Nifedipine, C. Diltiazem, D. Verapamil
D: Verapamil
Answer: D: Verapamil 
Explanation: Verapamil is phenylalkylamine which depresses the SA node and slows AV conduction. Others are less potent. But verapamil can cause constipation It is class 4 antiarrhythmic drug REF.Essentials of Pharmacology KD Tripathi 8th ed.