A 22-year-old woman complains of palpitations and has a regular heartbeat at a rate of 170/min, with a blood pressure of 110/70 mm Hg. The rate abruptly changes to 75/min after applying carotid sinus pressure. Which of the following is the most likely diagnosis?
sinus tachycardia paroxysmal atrial fibrillation paroxysmal atrial flutter paroxysmal supraventricular tachycardia (PSVT)