B

Both metoprolol (a β-blocker) and verapamil (a non-dihydropyridine calcium-channel blocker) slow sinus rate and AV-node conduction; their additive effects can precipitate profound bradycardia with varying degrees of AV block, which is well documented in case reports of the combination [Lee 1995, PMID 7785794; Wayne 1982, PMID 6127992].