MRCP2-2801

A 45-year-old man visits a sexual health clinic with a painless ulcer on the glans of his penis and inguinal lymphadenopathy. He had unprotected vaginal intercourse with a female partner within the last two weeks. He is prescribed doxycycline but returns the next day with a fever and rash. He is concerned about an allergy to doxycycline and consents to monitoring blood tests. What is the best course of action given the likely diagnosis?