A 23-year-old man is brought to the emergency department for sudden-onset palpitations and lightheadedness that began 30 minutes ago. He has no past medical history. Vital signs: T 36.8°C (98.2°F), BP 118/72 mm Hg, HR 190/min, RR 18/min, SpO2 99% on room air. He is anxious but alert and oriented. ECG shows an irregularly irregular wide-complex tachycardia with varying QRS morphology. The clinician suspects atrial fibrillation with preexcitation in Wolff-Parkinson-White (WPW) syndrome. Which of the following is the most appropriate immediate pharmacologic management?