What is the most appropriate investigation?

A 61-year-old woman was referred to the blood pressure clinic because of refractory hypertension. One year previously, her blood pressure, urea and electrolytes had been normal. Her current therapy included verapamil modified-release 240 mg daily and doxazosin 16 mg daily.

On examination, she was 1.63 m tall and weighed 90 kg with an elevated waist to hip ratio.

Her blood pressure was 182/94 mmHg supine.

Investigations:

serum sodium137 mmol/L (137C144)

serum potassium2.8 mmol/L (3.5C4.9)

serum creatinine79 µmol/L (60C110)

plasma renin activity (after 30 min supine)<1.1 pmol/mL/h (1.1C2.7)

plasma aldosterone (after 30 min supine)<135 pmol/L (135C400)

What is the most appropriate investigation?
A . 24-h urinary electrolytes
B . 24-h urine to assess free cortisol: cortisone ratio
C . analysis of the SCNN1B and SCNN1G genes
D . overnight dexamethasone suppression test
E . repeat renin and aldosterone concentrations after stopping verapamil for 2 weeks