**Core Concept:**

The question is based on the pharmacological management of atrial fibrillation in patients with rheumatic mitral stenosis who present with a fast ventricular rate. The treatment goal is to control the ventricular rate and maintain hemodynamic stability while minimizing side effects and potential drug interactions.

**Why the Correct Answer is Right:**

The correct answer (D) is beta-blocker therapy, specifically metoprolol. Beta-blockers are commonly used to control the ventricular rate in patients with atrial fibrillation. In the context of rheumatic mitral stenosis, beta-blockers are preferred over calcium channel blockers (option B) as they have less effect on the sinoatrial (SA) node, reducing the risk of bradycardia and SA node dysfunction.

In patients with rheumatic mitral stenosis, beta-blockers are beneficial as they:
1. Reduce the cardiac workload and oxygen demand, improving cardiac output and reducing symptoms like fatigue and dyspnea.
2. Lower the ventricular response rate, preventing excessive afterload on the left ventricle and reducing the risk of stroke in patients with atrial fibrillation.

**Why Each Wrong Option is Incorrect:**

A) Calcium channel blockers (verapamil) are not suitable in rheumatic mitral stenosis because they can cause bradycardia and SA node dysfunction, which may exacerbate the patient's condition.

B) Calcium channel blockers (diltiazem) can also cause bradycardia and SA node dysfunction, rendering them less suitable than beta-blockers in this context.

C) Digoxin is not the correct choice for ventricular rate control in rheumatic mitral stenosis because it primarily affects the atrial rate, not the ventricular rate. Additionally, it increases the risk of thromboembolism, which is undesirable in patients with atrial fibrillation and rheumatic mitral stenosis.

**Clinical Pearl:**

In the clinical scenario provided, metoprolol is the preferred option among the given choices, as it is a beta-blocker with a lower risk of bradycardia and SA node dysfunction compared to calcium channel blockers. Additionally, metoprolol has less potential to cause thromboembolism, which is crucial in patients with atrial fibrillation and rheumatic mitral stenosis.

**Explanation of the Question:**

The question tests the reader's understanding of the appropriate drug selection for ventricular rate control in a patient with rheumatic mitral stenosis and atrial fibrillation. The correct choice (metoprolol) needs to be selective and safe, as it prevents the negative side effects of bradycardia and SA node dysfunction caused by calcium channel blockers. Moreover, metoprolol has a lower risk of thromboembolism, which is vital in patients with atrial fibrillation and rheumatic mitral stenosis.