The most appropriate diagnostic test to perform next is to measure the plasma aldosterone-plasma renin activity ratio. This patient has resistant hypertension, defined as blood pressure that remains above goal despite concurrent use of three antihypertensive agents of different classes, one of which is a diuretic. Resistant hypertension may occur in as many as 10% of patients with hypertension. Although this patient is being treated with a diuretic, he has significant hypokalemia in the presence of treatment with an ACE inhibitor and potassium supplementation. This raises the possibility of primary hyperaldosteronism as a cause or contributing factor of his resistant hypertension. Appropriate initial evaluation for this diagnosis is measurement of the plasma aldosterone-plasma renin activity ratio. If positive, confirmatory testing is usually accomplished with intravenous salt loading, fludrocortisone suppression testing, or captopril testing. If confirmed, adrenal imaging is indicated to determine if hyperaldosteronism is due to a bilateral or unilateral cause. Mineralocorticoid receptor antagonists (such as spironolactone) are indicated for patients with a bilateral cause of primary hyperaldosteronism and those with a unilateral cause who refuse or are not candidates for surgery.

Dexamethasone suppression testing is used to evaluate for glucocorticoid excess. However, this patient is not taking exogenous glucocorticoids and has no physical examination findings (fat redistribution, striae) or laboratory studies (glucose metabolism abnormalities) suggesting Cushing syndrome. Therefore, testing for this possibility is not an appropriate next step in diagnosis.

Measurement of plasma metanephrines and catecholamines is used to evaluate for the possibility of pheochromocytoma, which classically presents with the triad of diaphoresis, headache, and tachycardia, none of which are present in this patient. In addition, pheochromocytoma would not explain the patient's hypokalemia.

Renal artery Doppler flow studies may be helpful in evaluating for renovascular hypertension. However, most cases of renovascular hypertension occur in patients over 50 years of age and are associated with atherosclerotic cardiovascular disease or evidence of functional impairment of the kidney, neither of which are apparent in this patient.