A 52-year-old man with a 10-year history of essential hypertension on atenolol and hydrochlorothiazide presents with recent-onset hypokalemia (K⁺ 3.1 mEq/L), metabolic alkalosis, and hypertension (BP 168/104 mmHg) refractory to current therapy. Plasma renin activity is suppressed and plasma aldosterone is elevated. Which investigation is most appropriate to localize the source of primary aldosteronism?