When the dental chair is moved to the upright position, a patient reports sudden dizziness and lightheadedness. What is the most likely cause?

Rapid postural change after prolonged supine/reclined positioning can cause orthostatic hypotension, especially in patients on antihypertensives, diuretics, or with autonomic dysfunction. Raising the chair slowly in stages, with a pause in the upright position before standing, helps prevent this.