Which parasitic organism is most often transmitted sexually?

Trichomonas vaginalis is the correct choice as it is a well-known protozoan parasite that is primarily transmitted through sexual contact. This organism is responsible for the infection known as trichomoniasis, which is one of the most common sexually transmitted infections (STIs) worldwide. The life cycle of Trichomonas vaginalis is closely associated with the urogenital tract, making sexual transmission the primary route for its spread.

In contrast, the other listed organisms do not primarily pivot on sexual transmission routes. For instance, Entamoeba gingivalis is often found in the oral cavity and can be transmitted through saliva, while Dientamoeba fragilis is believed to be transmitted through fecal-oral contamination rather than sexually. Diphyllobothrium latum, a fish tapeworm, is typically acquired through the ingestion of raw or undercooked fish, which is unrelated to sexual activity. Therefore, the focus on sexual transmission makes Trichomonas vaginalis the correct answer.