What is the most likely causative agent of a foul-smelling vaginal discharge and severe itching in a patient with highly motile, nucleated cells?

The symptoms described—a foul-smelling vaginal discharge and severe itching—along with the presence of highly motile, nucleated cells, strongly indicate an infection caused by Trichomonas vaginalis. This organism is a protozoan parasite known for causing trichomoniasis, a sexually transmitted infection characterized by these specific symptoms in women.

Trichomonas vaginalis is distinctive in clinical presentations due to its motility and the nature of the discharge it causes. The foul odor is typically associated with the production of compounds by the parasite and the resultant changes in the vaginal flora. Severe itching occurs as a response to the irritation and inflammation caused by the infection.

The other choices do not match the symptom profile or organism characteristics. Balantidium coli is primarily associated with intestinal infections and does not cause vaginal symptoms. Plasmodium falciparum is the causative agent of malaria and is not associated with vaginal discharge or itching. Giardia lamblia also causes gastrointestinal symptoms, not genitourinary symptoms, and is not characterized by foul-smelling discharge or motile nucleated cells typical of Trichomonas. Hence, the option of Trichomonas vaginalis accurately fits the clinical picture presented in the question.