Review Question - QID 214481

A 32-year-old man presents to his primary care provider with concerns of a diffuse rash. This patient mentions that he had a painless sore on the head of his penis 6 weeks ago that has since healed. He states that he is sexually active with both men and women, and rarely uses condoms. On physical examination, the patient has no sores on his genitals or elsewhere on his body. His rash is maculopapular and covers his palms and soles. Which of the following should the physician use to confirm the diagnosis for this patient?