A 28-year-old woman presents with obesity for 1 year. Physical examination reveals moon facies, increased acne, slight hirsutism, prominent vellus hair on the back, thickened nuchal fat pad, and BP of 150/100 mmHg. Laboratory tests show significantly elevated 24-hour urinary 17-OHCS and 17-KS levels, along with abnormal plasma cortisol rhythm. Which of the following is the most appropriate for definitive localization?