A 30-year-old woman presents to her gynecologist with complaints of vaginal itching and a frothy, yellow discharge. She also complains of painful urination. She admits to being sexually active with several men in the past two weeks. Cultures are negative for bacterial growth, but organisms are visible via a wet preparation on low power. The most likely causal agent is

  •   Trichomonas vaginalis
  •   Trichophyton rubrum
  •   Chlamydia trachomatis
  •  Candida albicans
  •  Giardia lamblia