<prompt> A 22-year-old woman complains of palpitations and has a regular heartbeat at a rate of 170min, with a blood pressure of 11070 mm Hg. The rate abruptly changes to 75min after applying carotid sinus pressure. Which of the following is the most likely diagnosis?sinus tachycardiaparoxysmal atrial fibrillationparoxysmal atrial flutterparoxysmal supraventricular tachycardia PSVT  <response>: The patient most likely has PSVT since the tachycardia terminates after carotid sinus massage CSM. CSM increases vagal tone parasympathetic which decreases AV nodal conduction and terminates AV node re-entry arrhythmias. Sinus tachycardia differs from PSVT tachycardia in that it does not start or stop abruptly. In PSVT, the QRS is usually narrow without clearly discernible P waves. A wide QRS in PSVT can result from a preexisting bundle branch block, or a functional bundle branch block secondary to the tachycardia. This can make the distinction from a ventricular arrhythmia quite difficult. 