You are answering a USMLE-style medical question. Question: A 25-year-old woman presents to her physician with a four month history of fatigue and weakness. The weakness has been progressive to the point where she cannot climb stairs and stand from a sitting position. She has only had one menstrual period in the last four months and has never been pregnant. She smokes a pack of cigarettes every day and does not take any medications. Her temperature is 98°F (36.7°C), blood pressure is 160/100 mmHg, pulse is 70/min, and respirations are 15/min. She is obese with a significant pannus. Abdominal striae are present. Her laboratory workup is notable for the following: Serum: Na+: 142 mEq/L Cl-: 102 mEq/L K+: 3.9 mEq/L HCO3-: 25 mEq/L BUN: 20 mg/dL Glucose: 314 mg/dL Creatinine: 1.1 mg/dL Ca2+: 10.1 mg/dL AST: 9 U/L ALT: 8 U/L 24-hour urinary cortisol: 470 µg (< 300 µg) Serum cortisol 30 µg/mL (5-23 µg/dL) Serum adrenocorticotropin-releasing hormone (ACTH) 2 pg/mL (> 5 pg/mL) A 48-hour high dose dexamethasone suppression trial shows that her serum cortisol levels do not decrease. What is the best next step in management? Options: A) MRI of the adrenal glands B) MRI of the chest C) Low dose dexamethasone suppression test D) Inferior petrosal sinus sampling Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.