A 64-year-old female has a blood pressure of 175/115 mm Hg while sitting and 180/110 mm Hg while standing and a pulse rate of 64/minute. A week later, her blood pressure is 190/120 mm Hg. Her blood pressure has been moderately well controlled in the past. She takes Propranolol 40 mg two times a day. She has a history of intermittent angina-like pain and has been a pack a day smoker for 30 years. ECG and chest x-ray studies reveal no abnormalities. What is the most likely cause of her worsening hypertension?