A 60-year-old man with a 20-year smoking history is brought to the physician by his daughter because of altered behavior over the past week. She says that her father’s personality has changed. He has had a productive cough over the past month, for which he is taking over-the-counter medications. He is not a drug addict, diabetetic or hypertensive. Physical examination shows multiple bruises on the skin and hyper-pigmented spots on the buccal mucosa and the back of his neck. His blood pressure is 160/90 mmHg, and pulse is 70/mm. Laboratory studies show random blood sugar of 224 mg/dL, serum sodium of 140 mEq/L, serum potassium of 2.0 mEq/L, and serum bicarbonate of 27 mEq/L. Which of the following tests is most likely to be abnormal in this patient?