A 28-year-old man from Mumbai presents to the STI clinic with a painless ulcer on the glans penis for 3 weeks. He reports a single sexual contact 4 weeks ago. On examination, the ulcer has a well-demarcated border, indurated base, and is non-purulent. Regional lymph nodes are enlarged, firm, and non-tender. Serum RPR is positive (1:16) and FTA-ABS is positive. Dark-field microscopy of ulcer exudate shows motile, spiral organisms. What is the most appropriate next step in management?