Which of the following arrhythmias would most likely require electrical cardioversion?

Atrial fibrillation is a supraventricular arrhythmia characterized by chaotic atrial depolarization, irregular ventricular response, loss of atrial kick, and risk of thromboembolism. Rapid ventricular rates ≥100 bpm cause hemodynamic instability, decreased cardiac output, hypotension, and increased myocardial oxygen demand. Rationale: A. Ventricular fibrillation is a life-threatening arrhythmia with disorganized ventricular electrical activity causing no cardiac output. It requires immediate defibrillation, not synchronized cardioversion. Absence of a pulse and collapse defines its emergency management pathway. B. First-degree heart block involves prolonged PR interval >0.20 seconds with intact conduction. It is usually asymptomatic and benign, requiring no acute intervention. Electrical cardioversion is not indicated because there is no hemodynamic compromise or unstable rhythm. C. Sinus tachycardia is a physiologic response to stressors such as hypovolemia, fever, or pain. Management focuses on treating the underlying cause rather than rhythm correction. Cardioversion is inappropriate since the rhythm originates from a normal sinus node mechanism. D. Atrial fibrillation with rapid ventricular rate results in irregularly irregular rhythm and compromised cardiac output. Synchronized cardioversion is indicated when patients are unstable or symptomatic. The goal is restoration of sinus rhythm and improved hemodynamic stability.