A 36-year-old woman presents with sudden-onset flaccid paralysis of all four limbs and dizziness. Physical examination: alert, facial flushing, generalized hyperpigmentation, acne vulgaris, perioral hirsutism, increased vellus hair growth. Thickened subcutaneous fat in the chest and abdomen, with 1.0cm wide purple striae on the lower abdomen. Fasting blood glucose (FBG) 10.2mmol/L, urinary 17-hydroxycorticosteroids (17-OHCS) 58μmol/24h (21mg/24h). After a low-dose dexamethasone suppression test, urinary 17-OHCS is 55.2μmol/24h (20.1mg/24h). Which of the following tests should be performed next?