A 48-year-old woman with a 10-year history of essential hypertension on amlodipine 5 mg daily presents with fatigue and dyspnea on exertion. Her blood pressure is 165/105 mmHg. Laboratory tests show: serum sodium 128 mEq/L, serum osmolality 265 mOsm/kg, urine osmolality 580 mOsm/kg, and urine sodium 65 mEq/L. Plasma renin activity is elevated at 4.2 ng/mL/hr (normal 0.5–1.6). What is the most likely explanation for her hypertension and hyponatremia?