Review Question - QID 216396

A 67-year-old woman presents from a nursing home with altered mental status. She was last known to be normal the night before. The patient has dementia at baseline and is minimally interactive. This morning, she was found to be obtunded. No further history was provided from the nursing home, and the patient is unable to provide any history. Her temperature is 95.0°F (35.0°C), blood pressure is 84/54 mmHg, pulse is 50/min, respirations are 9/min, and oxygen saturation is 98% on room air. Physical exam reveals an obtunded woman who only withdraws her extremities to pain. Her skin is cool and pale, and her hair is thin. There are no signs of trauma noted on exam. Laboratory values are notable for a whole blood sodium of 120 mEq/L. The patient is given several intravenous boluses of fluids and her blood pressure improves to 100/60 mmHg. Which of the following is most likely to confirm the underlying cause of this patient's symptoms?