A 38-year-old woman with a 10-year history of recurrent syncope, orthostatic intolerance, and postprandial hypotension is suspected of having autonomic dysfunction. Supine and standing vital signs show a drop of 25 mmHg systolic and 15 mmHg diastolic 3 minutes after standing, with symptoms of dizziness. Which investigation is most appropriate to confirm parasympathetic and sympathetic dysfunction?