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.
On dark-field examination, T pallidum (the spirochete that causes syphilis) is a thin, delicate organism with tapering ends and 6-14 spirals. When dark-field examination is not possible, direct fluorescent antibody tests are used.
toxoplasmosis
tetanus
syphilis
smallpox
Infection