A 26-year-old lady, complains to her gynaecologist of discomfo during intercourse. Pelvic examination demonstrates a frothy, yellow-green vaginal discharge with a strong odour and small, red, ulcerations of the vaginal wall. A wet mount preparation demonstrates motile, flagellated protozoa. Which of the following is the most likely causative organism?

Trichomonas vaginalis, the causative organism of trichomoniasis, is a single-celled protozoan parasite with a predilection for infecting the vagina. The organism can be identified with intra-office wet mount preparations, cultured, or recognized on Pap smears (although this should be confirmed by culture, as the Pap smear is not a paicularly reliable method). Candida albicans is a spore- and hyphae -forming fungus that can affect the vagina, often producing a discharge with a white, "cheesy" appearance. Entamoeba histolytica is an amoeba that can infect the gut. Giardia lamblia is a flagellated parasite that can infect the gut.