A 52-year-old man has a blood pressure of 162/98 mmHg despite adequate doses of perindopril, amlodipine and hydrochlorothiazide, with good adherence confirmed. He is asymptomatic, with no episodes of headache, sweating or palpitations, and examination is unremarkable apart from the elevated blood pressure with no abdominal bruit. Blood tests show potassium 3.1 mmol/L (3.5-5.2), sodium 143 mmol/L (135-145) and a normal eGFR. Which one of the following investigations is most appropriate next?
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