MS-0849

A 39-year-old woman presents to the endocrine clinic after being referred by her GP due to a blood pressure reading of 178/101 mm Hg. Upon blood tests, it is discovered that she has hypernatremia and hypokalaemia, along with an elevated aldosterone level. An inconclusive CT scan of the abdomen has been performed to determine if there is an adenoma present.

What is the most suitable investigation to identify if one of the adrenal glands is producing an excess of hormones?