A 22-Year-Old Woman Comes to the Physician Because of Yellow

A 22-year-old woman comes to the physician because of yellow vaginal discharge, itching, and pain with urination for the past 6 days. She has been sexually active with multiple male partners over the past year and uses condoms inconsistently; she has been taking an oral contraceptive for the past 2 years. She has no history of serious illness or sexually transmitted diseases. Pelvic examination shows erythema of the vulva and vagina and foul-smelling, frothy, yellow-green vaginal discharge. Which of the following is the best test to confirm the diagnosis in this patient?

A) Cervical cytology
B) Gram stain
C) Potassium hydroxide testing
D) Urine culture
E) Vaginal pH
F) Wet mount saline microscopy