This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
[{'content': '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. \nSelect and explain. \nA. toxoplasmosis, B. tetanus, C. syphilis, D. smallpox', 'role': 'user'}, {'content': 'Answer: C \nExplanation: 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.', 'role': 'assistant'}]