A 38-year-old woman presents with progressive proximal muscle weakness, easy bruising, and amenorrhea for 6 months. Examination reveals central obesity, purple striae, and hypertension (160/95 mmHg). Initial screening with 24-hour urinary free cortisol is elevated at 180 µg/24h (normal <50). Low-dose dexamethasone suppression test (1 mg overnight) shows cortisol of 18 µg/dL. What is the most appropriate next step in management?