A 28-year-old woman comes to the clinic for her third episode of vulvovaginal itching and burning in the past 6 months. She reports thick, white vaginal discharge without foul odor. She recently completed a 7-day course of oral antibiotics for sinusitis. She is sexually active with one partner and does not use condoms. She has no fever or pelvic pain. Pelvic examination shows vulvar erythema and adherent white plaques on the vaginal walls. Vaginal pH is 4.0. A saline wet mount shows no motile organisms. Which of the following underlying conditions should be evaluated in this patient to reduce risk of recurrent infection?