Review Question - QID 108694

A 65-year-old woman is brought to the emergency room by her family with a 2 week history of confusion and change in behavior. Her family states that she has become increasingly irritable and confused as well as aggressive toward strangers. In addition to her altered mental status, her family also endorses recent episodes of abdominal pain and watery diarrhea. Her medications include hydrochlorothiazide, enalapril, loperamide, and a calcium supplement. She has no history of recent travel outside the United States. Her temperature is 99.5°F (37.5°C), pulse is 112/min, blood pressure is 100/70 mmHg, respirations are 18/min, and oxygen saturation is 93% on room air. A physical exam reveals a thin, ill-appearing woman, sinus tachycardia, and hyperactive bowel sounds. A rash with purple discoloration and scaling is noted around her neck, upper chest, hands, and feet. A chest radiograph shows clear lung fields bilaterally. An echocardiogram shows thickening of the right ventricular endocardium with mild tricuspid stenosis. Which of the following is the most appropriate next diagnostic step for this patient?