Review Question - QID 212788

A 54-year-old man comes to the clinic for complaints of fever and myalgia for the past 3 days. The symptoms appeared around the time when he started having non-bloody diarrhea, which the patient attributed to a bad oyster. However, the patient grew concerned when the diarrhea and fever persisted. He reports 2 documented episodes of fever yesterday and endorses fatigue, myalgia, palpitations, and night sweats. He denies headaches, chest pain, shortness of breath, abdominal pain, or urinary symptoms. Per the patient, he has been relatively healthy except for an episode of the flu 2 weeks ago. He works as a prison guard and currently has multiple sexual partners with inconsistent condom use. His temperature is 101°F (38°C), blood pressure is 118/70 mmHg, pulse is 103/min, respirations are 18/min, and SpO2 is 99% on room air. A physical examination demonstrates bilateral axillary lymphadenopathy, a rash on the torso, and diffuse muscle tenderness. A chest radiograph is obtained and is shown in Figure A. What is the best initial test to diagnose this patient’s condition?