A 51-year-old woman is evaluated following an emergency department visit for abdominal pain. The pain spontaneously resolved. A CT scan in the emergency department revealed an incidentally discovered 1.4-cm left adrenal nodule with smooth borders and low attenuation and vascularity. She is otherwise healthy and takes no medications, On physical examination, blood pressure is 120/80 mm Hg, pulse rate is 60/min, and respiration rate is 14/min. The remainder of the physical examination is normal. A comprehensive metabolic profile, including electrolytes, is normal. Which of the following diagnostic tests should be done next?