question: Q:A 45-year-old woman presents to her physician with a four-month history of headache. Her headache is nonfocal but persistent throughout the day without any obvious trigger. She was told that it was a migraine but has never responded to sumatriptan, oxygen, or antiemetics. She takes amlodipine for hypertension. She does not smoke. She denies any recent weight loss or constitutional symptoms. Her temperature is 98°F (36.7°C), blood pressure is 180/100 mmHg, pulse is 70/min, and respirations are 15/min. She is obese with posterior cervical fat pads and central abdominal girth. Her neurological exam is unremarkable. In her initial laboratory workup, her fasting blood glucose level is 200 mg/dL. The following additional lab work is obtained and is as follows:
  
  Serum:
  Na+: 142 mEq/L
  Cl-: 102 mEq/L
  K+: 4.1 mEq/L
  HCO3-: 24 mEq/L
  BUN: 20 mg/dL
  Glucose: 135 mg/dL
  Creatinine: 1.3 mg/dL
  Ca2+: 10.0 mg/dL
  AST: 8 U/L
  ALT: 8 U/L
  24-hour urinary cortisol: 500 µg (reference range < 300 µg)
  Serum cortisol: 25 µg/mL (reference range 5-23 µg/dL)
  24-hour low dose dexamethasone suppression test: Not responsive
  High dose dexamethasone suppression test: Responsive
  Adrenocorticotropin-releasing hormone (ACTH): 20 pg/mL (5-15 pg/mL)
  
  Imaging reveals a 0.5 cm calcified pulmonary nodule in the right middle lobe that has been present for 5 years but an otherwise unremarkable pituitary gland, mediastinum, and adrenal glands. What is the best next step in management?? 
  {'A': 'Repeat high dose dexamethasone suppression test', 'B': 'Inferior petrosal sinus sampling', 'C': 'Pituitary resection', 'D': 'CT-guided biopsy of the pulmonary nodule', 'E': 'Pulmonary nodule resection'},