A 72-year-old man presents to the emergency department with recurrent episodes of syncope triggered by shaving and wearing tight collars. On examination, carotid auscultation reveals no bruits. Carotid sinus massage with continuous ECG monitoring shows a 3-second asystolic pause and a 40 mmHg drop in systolic blood pressure. The structure marked **A** in the diagram (carotid sinus baroreceptors) is demonstrating exaggerated sensitivity. Which of the following best describes the mechanism of syncope in this patient?