**Core Concept**
Diltiazem is a non-dihydropyridine calcium channel blocker that primarily affects the L-type calcium channels in the heart, leading to a reduction in the inward calcium current. This ultimately results in a decrease in heart rate and contractility, as well as a prolongation of the atrioventricular (AV) node conduction time.

**Why the Correct Answer is Right**
Diltiazem works by selectively blocking the L-type calcium channels, which are responsible for the plateau phase of the cardiac action potential. By inhibiting these channels, diltiazem reduces the influx of calcium ions into the cardiac myocytes, leading to a decrease in heart rate and contractility. Additionally, the blockade of L-type calcium channels prolongs the AV node conduction time, resulting in a delay in the transmission of electrical impulses from the atria to the ventricles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the mechanism of action of diltiazem. Although beta-blockers do slow heart rate and reduce contractility, they do so through a different mechanism involving the blockade of beta-adrenergic receptors.

**Option B:** This option is incorrect because it does not accurately describe the electrocardiographic effects of diltiazem. While diltiazem does prolong the PR interval, it does not have a significant effect on the QRS complex or the QT interval.

**Option C:** This option is incorrect because it does not accurately describe the mechanism of action of diltiazem. Although verapamil is another calcium channel blocker, it has a different pharmacological profile and is more potent in its effects on the AV node.

**Option D:** This option is incorrect because it does not accurately describe the mechanism of action of diltiazem. Although diltiazem does have some effects on peripheral vessels, its primary mechanism of action is through the blockade of L-type calcium channels in the heart.

**Clinical Pearl / High-Yield Fact**
Diltiazem is often used to treat hypertension and angina pectoris, and its use is particularly beneficial in patients with Wolff-Parkinson-White syndrome, where it can help to slow the conduction velocity through the bypass tract and reduce the risk of arrhythmias.

**Correct Answer:** B.