A 37-yeаr-оld wоmаn visited her gynecоlogist becаuse of a swelling in her vulva. She told the physician that the swelling has been enlarging slowly. On examination, the physician palpated a nontender circumscribed fluid-like mass protruding into the vestibule of the vagina, posterolateral to the vaginal orifice. There was no history of gonorrhea. Which of the following is most likely correct?