A 28-year-old woman presents to the emergency department with palpitations and chest discomfort lasting 2 hours. She reports sudden onset while at rest. Vital signs: HR 180 bpm, BP 110/70 mmHg, RR 18/min. Physical examination reveals a regular tachycardia with no murmurs. ECG shows a narrow complex tachycardia with a rate of 180 bpm; P waves are not clearly visible. After vagal maneuvers fail, intravenous adenosine 6 mg is administered. The rhythm abruptly terminates, followed by sinus rhythm at 72 bpm with a normal PR interval and QRS duration. What is the most likely diagnosis?