Final answer:
The anaerobic protozoan called Trichomonas vaginalis is a common resident of the female vagina and male urethra, and can cause the common STI known as trichomoniasis.
Step-by-step explanation:
The anaerobic protozoan frequently found as a normal inhabitant of the vagina in females and of the urethra in many males is Trichomonas vaginalis. This flagellated protozoan causes trichomoniasis, which is a common sexually transmitted infection (STI). Trichomonas vaginalis is known for its undulating membrane and amoeboid shape when adhering to vaginal cells. Unlike other protozoans that undergo encystment, this protozoan exists solely as a trophozoite and is equipped with cell-surface virulence factors such as lipoglycans and tetraspanins which assist in cell adhesion, motility, and invasion. It's important to note that Trichomonas vaginalis, while part of the normal microbiota, can proliferate and cause symptoms when the normal balance of vaginal or urethral microorganisms is disrupted.