MRCP2-0294

A 25-year-old female presents to cardiology outpatient clinic with recurrent episodes of drowsiness, somnolence, and occasional syncope. She has been diagnosed with sinus bradycardia and is scheduled for pacemaker insertion. Two weeks after the procedure, she reports persistent symptoms of dizziness and occasional palpitations with pulsation in the neck. On examination, her vital signs show a heart rate of 40/min regular, blood pressure of 80/40 mmHg, and oxygen saturation of 99% on air. An ECG reveals 1:1 AV block and bradycardia. What is the most likely diagnosis?