Based on the complaints of the patient, he is most likely suffering from orthostatic hypertension. Orthostatic hypertension refers to a condition characterized by drop in the blood pressure of 20 mmHg or more within 3 minutes of standing compared to the blood pressure of the patient in sitting or lying position. Starting on a new ACE inhibitor—an antihypertensive—could explain this finding. Allopurinol (gout), metformin (diabetes), or azithromycin (bacterial infection) are not known to cause orthostatic hypertension.