A 42-year-old woman comes to the physician because of vaginal itch and discharge, dysuria, and dyspareunia. These symptoms have been steadily worsening over the past 3 days. Pelvic examination reveals an erythematous vagina and a thin, green, frothy vaginal discharge with a pH of 6. Microscopic examination of the discharge demonstrates the presence of a pear-shaped, motile organism. Which of the following is the most likely pathogen? (A) Candida albicans (B) Gardnerella vaginalis (C) Herpes simplex virus (D) Treponema pallidum (E) Trichomonas vaginalis