Review Question - QID 108570

A 45-year-old man with history of IV drug use presents to the outpatient clinic with a history of fever for 5 days. He also endorses sweats, headache behind the eyes, sore throat, muscle and joint pain, and a new rash seen in Figure A. He reports that he traveled to Africa last year and returned from a vacation to Thailand 10 days ago. His immunizations are up to date, although he missed the pre-departure travel medicine consult before his trip to Thailand. His temperature is 101.5°F (38.6°C), blood pressure is 125/80 mmHg, pulse is 105/min, and respirations are 14/min. Laboratory testing reveals the following:

Leukocyte count: 2,400/mm^3
Hemoglobin: 13.4 g/dL
Hematocrit: 40%
Mean corpuscular volume: 81 µm^3
Platelets: 92,000/mm^3

Serum:
AST: 112 IU/L
ALT: 69 IU/L
ALP: 78 IU/L
Bilirubin: 0.9 mg/dL

Which of the following is the diagnostic test of choice for this disease?