A 24-year-old woman presents with a 3-month history of intermittent vaginal discharge. She describes the discharge as clear and stretchy, similar to egg white, occurring every few weeks without a noticeable odor. Her menstrual cycles are regular every 28 days, associated with mild cramping and 1 day of increased fatigue; her last menstrual period was 2 weeks ago. Medical history is notable for a recent urinary tract infection treated with ciprofloxacin. She became sexually active with a new partner 6 months ago and does not use barrier contraception. Vital signs are within normal limits. Abdominal examination is soft and nontender. Pelvic examination reveals a clear, thin discharge in the vaginal vault without vulvar erythema or irritation and no cervical motion or adnexal tenderness. A wet mount of a vaginal swab is shown in Figure A. Which of the following is the most likely underlying etiology of this patient’s condition?