**Core Concept**
Paroxysmal supraventricular tachycardia (PSVT) is a type of abnormal heart rhythm characterized by a rapid heart rate originating from above the ventricles. The drug of choice in PSVT is typically a medication that can terminate the arrhythmia by blocking the AV node.

**Why the Correct Answer is Right**
The correct answer is a beta-blocker, specifically adenosine. Adenosine works by transiently blocking the AV node, which allows the heart to return to its normal sinus rhythm. This is because adenosine binds to A1 receptors on the AV node, causing a brief period of AV block. This mechanism allows the heart to reset and return to a normal rhythm. Other beta-blockers like metoprolol can also be used but are not the first-line treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** Digoxin is a cardiac glycoside that increases the force of contraction of the heart muscle but is not the first-line treatment for PSVT. It can actually worsen the condition by increasing the heart rate.
**Option B:** Quinidine is an anti-arrhythmic medication that can be used for various types of arrhythmias but is not the preferred treatment for PSVT.
**Option C:** Procainamide is another anti-arrhythmic medication that can be used for PSVT but is not the first-line treatment due to its potential side effects.

**Clinical Pearl / High-Yield Fact**
When treating PSVT, it's essential to remember that vagal maneuvers such as carotid massage or the Valsalva maneuver can also terminate the arrhythmia by stimulating the vagus nerve and causing a brief period of AV block.

**Correct Answer: A. Adenosine. Adenosine works by transiently blocking the AV node, allowing the heart to return to its normal sinus rhythm.**