A 42-year-old man with a 3-year history of hypertension (currently on amlodipine 5 mg daily) presents with worsening fatigue and dyspnea on exertion. On examination, blood pressure is 156/98 mmHg, heart rate 88/min, with a pulsatile abdominal mass palpable above the umbilicus. Serum creatinine is 2.2 mg/dL (baseline 1.0 mg/dL 1 year ago), and abdominal CT angiography shows a 5.5 cm infrarenal aortic aneurysm with eccentric thrombus. Serum aldosterone is 18 ng/dL and plasma renin activity is 2.1 ng/mL/h (both normal range). Which secondary cause of hypertension should be suspected?