A Pap smear from a 30-year-old woman demonstrates protozoal parasites. These organisms are likely to be:

Trichomonas vaginalis (note "vagina" in the name) is the only organism listed to primarily affect the genital tract rather than the intestinal tract. The vaginitis it causes is characterized by a frothy, yellow discharge. The organism can be identified in Pap smears, or, more reliably, by special culture techniques that are becoming more widely available. Infection in men is usually asymptomatic, but prostatitis and urethritis can also occur. Treatment with metronidazole is indicated for both the patient and the sexual paner(s). The other organisms listed cause gastrointestinal disease.