Which of the following medications is not classified as a Vaughan-Williams Class IV drug?

Atenolol is indeed not classified as a Vaughan-Williams Class IV drug. The Vaughan-Williams classification system categorizes antiarrhythmic drugs into different classes based on their electrophysiological effects on cardiac tissue.

Class IV agents specifically include calcium channel blockers, which primarily affect the atrioventricular (AV) node and help to manage conditions such as supraventricular tachycardia by slowing conduction. Verapamil and diltiazem are both recognized as Class IV medications due to their calcium channel blocking properties, which influence heart rate and rhythm.

Atenolol, on the other hand, is a beta-blocker, classified as a Class II antiarrhythmic. Beta-blockers work by blocking the effects of adrenaline on beta-adrenergic receptors, leading to a decrease in heart rate and myocardial contractility. Hence, it does not fall under the Class IV category but is rather used for similar therapeutic objectives in the management of arrhythmias and other cardiovascular conditions.