A 28 year old lady presents with weight gain (10 kg over a period of 3 years), and has oligomenorrhoea followed by amenorrhoea for 8 months, her blood pressure is 160/100 mm of Hg. Which of the following investigation is most useful in making the diagnosis?

Lady in the question stem is showing features of Cushings syndrome. It is diagnosed by an increased 24-hour urinary free coisol excretion in three separate collections, failure to appropriately suppress morning coisol after overnight exposure to dexamethasone and evidence of loss of diurnal coisol secretion with high plasma coisol levels at mid-night, the time of physiologically lowest secretion. Ref: Harrison's Internal Medicine, 18th Edition, Chapter 342