Cardiovascularcommon
A 38-year-old man is reviewed because his blood pressure remains elevated despite perindopril 10 mg and amlodipine 10 mg daily. He feels well, sleeps soundly without snoring, drinks little alcohol, and reports no episodes of headache, sweating or palpitations. Repeated office readings average 158/98 mmHg, confirmed on 24-hour ambulatory monitoring. Examination shows a body mass index of 26 kg/m², equal blood pressures in both arms, normal femoral pulses and no abdominal bruit; there are no striae or easy bruising. Serum sodium is 143 mmol/L (135–145), potassium 3.2 mmol/L (3.5–5.2) and creatinine 74 µmol/L (60–110). Which one of the following investigations is most likely to identify the underlying cause?