Which sexually transmitted infection is caused by protozoa?

Trichomoniasis is caused by the protozoan parasite Trichomonas vaginalis, which primarily infects the urogenital tract in both men and women. This infection is transmitted primarily through sexual contact, making it a sexually transmitted infection (STI). Symptoms can include genital discomfort, discharge, and itching, although it is possible for individuals to be asymptomatic. Diagnosis typically involves laboratory testing of vaginal or urethral secretions.

The other options pertain to different conditions: Scabies is caused by a mite and leads to intense itching and rash; Chancroid is caused by the bacterium Haemophilus ducreyi and results in painful ulcers; and Pediculosis refers to lice infestations, which are not classified as STIs even though they can be transmitted through close physical contact. Thus, Trichomoniasis is the only condition listed that is specifically recognized as being caused by a protozoan pathogen, affirming its classification as an STI.