A 24-year-old with a 1-week history of thin, yellow-green discharge and numerous WBCs is most likely to have which sexually transmitted infection?

The symptoms presented by the patient—a thin, yellow-green discharge accompanied by a significant presence of white blood cells—are characteristic of trichomoniasis. This sexually transmitted infection is caused by the protozoan Trichomonas vaginalis and is known for causing a frothy, often yellow-green discharge, as well as vaginal inflammation and discomfort.

In the context of a sexually active individual, the combination of discharge and the elevated white blood cell count indicates an infectious process, and trichomoniasis is particularly notable for these findings. The discharge typically has a distinct odor which can also help in diagnosis.

Other infections like gonorrhea and chlamydia can present with discharge as well, but the quality and color of the discharge in those cases differ. Bacterial vaginosis, while also involving discharge, is caused by an imbalance in the vaginal flora rather than a typical sexually transmitted pathogen, and it usually presents with a characteristic odor rather than a distinctly colored discharge.

Thus, given the specific parameters of thin, yellow-green discharge and the associated symptoms, trichomoniasis emerges as the most likely diagnosis in this clinical scenario.