A 37-year-old woman presents to the emergency room after falling down some stairs at work. She reports no pre-monitory symptoms prior to the fall or any loss of consciousness. Recently she has noticed persistent symptoms of fatigue, muscle weakness, and unexpected weight gain. She has no past medical history and is not taking any medications.On examination, the blood pressure is 164/92 mm Hg and heart rate is 84/min. There are multiple skin bruises, facial fullness, and truncal obesity with red "stretch marks." Muscle strength in the proximal muscles is 4/5 and the reflexes are normal. Which of the following is the most appropriate initial diagnostic test?