A 38-year-old woman presents with hypertension (BP 165/105 mmHg), hypokalemia (K⁺ 2.8 mEq/L), and metabolic alkalosis (pH 7.48, HCO₃⁻ 32 mEq/L). Plasma aldosterone is markedly elevated (45 ng/dL; normal <15) with suppressed plasma renin activity (0.2 ng/mL/hr; normal 0.5–2.5). A contralateral adrenal CT shows a 1.2 cm nodule. Which feature best distinguishes primary aldosteronism from secondary aldosteronism?