**Core Concept:**
The effective refractory period (ERP) is the duration of time during which a cardiac cell cannot generate an action potential in response to a stimulus. In the context of the atrioventricular (AV) node, the ERP determines the time it takes for the impulse to travel from the atria to the ventricles before it is blocked and needs to restart. The AV node acts as a safety mechanism to prevent rapid atrial contractions from triggering ventricular contractions too frequently.

**Why the Correct Answer is Right:**
The drug that increases the ERP of the AV node is **Doxazosin**. Doxazosin is an alpha-1 blocker commonly prescribed for the treatment of hypertension. It acts by blocking alpha-1 receptors on the atrial and ventricular myocytes, thereby reducing the rate at which the receptor returns to its resting state after being stimulated. This prolongs the refractory period, including the ERP of the AV node.

**Why Each Wrong Option is Incorrect:**
A. **Digoxin**: Digoxin is a cardiac glycoside that primarily affects the sinoatrial (SA) node and increases the heart rate. It does not directly influence the ERP of the AV node.

B. **Beta-blockers**: Beta-blockers are used to reduce heart rate and blood pressure, but they do not primarily affect the AV node's ERP.

C. **Calcium channel blockers**: Calcium channel blockers are primarily used to treat hypertension and angina by blocking calcium channels in cardiac myocytes. They do not specifically target the AV node's ERP.

**Clinical Pearl:**
In the context of managing patients with rapid heart rates (e.g., in supraventricular tachycardia), doxazosin can be used to slow down the heart rate by blocking alpha-1 receptors, thereby indirectly affecting the AV node and its refractory periods. However, it is essential to consider other antiarrhythmic agents like beta-blockers, calcium channel blockers, or digoxin depending on the specific tachycardia and the patient's overall condition. Always consult a cardiologist before initiating any antiarrhythmic medication.

**Correct Answer Explanation:**
Doxazosin increases the refractory period of the AV node, making it a suitable drug for managing patients with rapid heart rates like those in supraventricular tachycardia. However, other antiarrhythmic agents may be required depending on the specific type of tachycardia and the patient's overall condition. Always consult with a cardiologist before administering any antiarrhythmic medication.