Review Question - QID 109933

A 31-year-old man presents to the general health clinic for a routine checkup. He admits to being in good health and has no medical concerns. He drinks alcohol regularly and smokes marijuana occasionally. He has been sexually active for the past 10 years and has had over 25 sexual partners, both male and female. He uses barrier protection most of the time. His temperature is 99.5°F (37.5°C), blood pressure is 127/88 mmHg, pulse is 80/min, respirations are 17/min, and oxygen saturation is 98% on room air. Physical exam demonstrates multiple skin lesions on the patient's arms and around his anus that resemble the finding in Figure A. Which of the following is the best confirmatory test for the most likely underlying diagnosis in this patient?