**Core Concept**
The prolongation of the QT interval on an electrocardiogram (ECG) is a critical concern in clinical practice, as it increases the risk of potentially fatal arrhythmias, such as torsades de pointes. This phenomenon is often associated with drugs that affect the repolarization phase of the cardiac action potential.

**Why the Correct Answer is Right**
The correct answer is a class III antiarrhythmic agent that blocks potassium channels in the heart, thereby prolonging the action potential duration and, consequently, the QT interval. This class of drugs includes sotalol, amiodarone, and dofetilide, which are commonly used to treat supraventricular tachycardias and atrial fibrillation. The mechanism of action involves the inhibition of the slow component of the delayed rectifier potassium current (IKs), which leads to a prolongation of the QT interval.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not typically cause QT prolongation. Instead, it is a beta-blocker commonly used to treat hypertension, angina, and certain arrhythmias.
**Option B:** This option is incorrect because it is an antipsychotic medication that can cause QT prolongation as a side effect, but it is not primarily used to treat cardiac arrhythmias.
**Option C:** This option is incorrect because it is a calcium channel blocker that is used to treat hypertension, angina, and certain arrhythmias, but it does not typically cause QT prolongation.

**Clinical Pearl / High-Yield Fact**
When prescribing medications, it is essential to consider the potential for QT prolongation, especially in patients with pre-existing cardiac conditions or those taking other medications that may increase the risk of arrhythmias.

**Correct Answer: B. Quetiapine.**