A 24 year old, non-gravid, sexually active woman complains o…

A 24 year old, non-gravid, sexually active woman complains of vaginal itching.  She has experienced burning when she urinates but no fever or back pain.  She appears well and her abdominal examination is normal, with no costovertebral angle tenderness.  Pelvic examination reveals a thin, gray-white, moderately increased, vaginal discharge.  Clue cells are present on microscopy of vaginal secretions.  You also note an amine odor on KOH examination.  You correctly diagnose this young woman with bacterial vaginosis.  Which of the following agents is the most appropriate treatment?