Behavioral Science & Ethics USMLE Step 1 Practice Question

A 77-year-old woman with a history of mild cognitive impairment is brought to the emergency department by her daughter with acute-onset confusion and agitation over the past 4 hours. The daughter reports the patient is disoriented to time and place, intermittently incoherent, and unusually irritable—a marked change from her baseline. Vital signs show temperature 38.2°C (100.8°F), heart rate 102/min, respiratory rate 16/min, and blood pressure 138/68 mmHg. Physical examination reveals mild suprapubic tenderness and costochondral angle tenderness bilaterally. Cognitive testing shows she is oriented only to person. Which of the following is the most appropriate initial diagnostic test?