Review Question - QID 108708

A 72-year-old man presents to his primary care physician complaining of increasing difficulty sleeping over the last 3 months. He reports waking up frequently during the night because he feels the urge to move his legs and he has a similar feeling when watching television before bed. The urge is relieved by walking around or rubbing his legs. The patient’s wife also notes that she sometimes sees him moving his legs in his sleep and is sometimes awoken by him. Due to his recent sleep troubles, the patient has started to drink more coffee throughout the day to stay awake and reports having up to 3 cups daily. The patient has a past medical history of hypertension and obesity but states that he has lost 10 pounds in the last 3 months without changing his lifestyle. He is currently on hydrochlorothiazide and a multivitamin. His last colonoscopy was when he turned 50 and he has a family history of type 2 diabetes and dementia. At this visit, his temperature is 99.1°F (37.3°C), blood pressure is 134/81 mmHg, pulse is 82/min, and respirations are 14/min. On exam, his sclerae are slightly pale. Cardiovascular and pulmonary exams are normal and his abdomen is soft and non-tender. A neurologic exam reveals 2+ reflexes in the bilateral patellae and 5/5 strength in all extremities. Which of the following is most likely to identify the underlying etiology of this patient's symptoms?