Endocrine Pharmacology USMLE Step 1 Practice Question

A 45-year-old woman presents with a 9-month history of progressive hirsutism, proximal muscle weakness, and central obesity with violaceous striae on her abdomen. She reports easy bruising, irregular menses, and mild cognitive difficulties. Vital signs show BP 158/94 mmHg, HR 82/min, RR 16/min, and temperature 37°C. Physical examination reveals a dorsocervical fat pad, facial plethora, and bilateral proximal lower extremity weakness. Laboratory findings include a fasting glucose of 118 mg/dL, serum potassium of 3.2 mEq/L, and morning serum cortisol of 18 mcg/dL following overnight low-dose dexamethasone suppression testing (normal less than 5 mcg/dL after suppression). She currently takes no exogenous glucocorticoids. Which drug is most appropriate for assessing ACTH-driven endogenous cortisol suppression in this diagnostic evaluation?