A 28-year-old man presents with a new genital ulcer on his penis that is painless. He is sexually active and noticed the lesion 1 week ago. The ulcer is 1 cm in size, has an eroded base, and an indurated margin. Dark-field examination of the ulcer fluid confirmed the diagnosis.For the above patient, select the most likely infecting organism.
toxoplasmosis tetanus syphilis smallpox