## Why "Atrial fibrillation; metoprolol" is right The **irregularly irregular R-R intervals** (marked **B**) are the pathognomonic ECG finding of atrial fibrillation (AF), the most common sustained arrhythmia. This chaotic ventricular response results from random conduction of atrial fibrillatory waves (350–600 bpm) through the AV node. In a patient without heart failure, beta-blockers (metoprolol, bisoprolol) are the first-line rate-control agents per Harrison 21e Ch 247 and ESC AF Guidelines 2024. The target resting heart rate is typically <110 bpm (lenient control per RACE II trial). ## Why each distractor is wrong - **Atrial flutter; diltiazem**: Atrial flutter produces a *regularly regular* ventricular rhythm (sawtooth baseline), not irregularly irregular. Diltiazem is a non-DHP CCB reserved for patients with contraindications to beta-blockers or those with HFrEF (where it is avoided). - **Sinus tachycardia with frequent ectopy; digoxin**: Sinus tachycardia has visible P waves and regular intervals between sinus beats; ectopy produces isolated premature beats, not sustained irregularly irregular rhythm. Digoxin is a weak rate-control agent used as add-on therapy, especially in heart failure, not as monotherapy in this scenario. - **Multifocal atrial tachycardia; verapamil**: MAT occurs in COPD/critical illness and shows ≥3 different P-wave morphologies with irregular intervals; it is much rarer than AF. Verapamil is a non-DHP CCB contraindicated in HFrEF and less effective than beta-blockers for AF rate control. **High-Yield:** Irregularly irregular rhythm = AF until proven otherwise; beta-blockers are first-line rate control in non-HFrEF patients; CHA₂DS₂-VASc ≥2 (men) or ≥3 (women) mandates anticoagulation. [cite: Harrison 21e Ch 247; ESC AF Guidelines 2024]
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