Concept:Syphilis is a sexually transmitted infection caused by the bacterium *Treponema pallidum*.Explanation:• Gonorrhea is caused by *Neisseria gonorrhoeae*.• Leprosy is caused by *Mycobacterium leprae*.• Tuberculosis is caused by *Mycobacterium tuberculosis*.• Syphilis is caused by *Treponema pallidum*.Answer:D. Syphilis