A 28-year-old woman presents to the emergency department with palpitations and chest discomfort lasting 2 hours. She reports a sudden onset of rapid regular heartbeat while sitting at her desk. On examination, BP is 118/76 mmHg, HR 180/min, regular. Physical examination reveals cannon a waves in the JVP. A 12-lead ECG shows a narrow complex tachycardia with a rate of 180/min; P waves are not clearly visible. Carotid massage is attempted but produces no change in heart rate. An intravenous bolus of adenosine 6 mg is given, resulting in immediate termination of the arrhythmia and restoration of normal sinus rhythm. What is the most likely diagnosis?