A 41-year-old nulliparous woman, at 15 weeks' gestation comes to the emergency department because of an 8-hour history of light vaginal bleeding. She had a spontaneous abortion at 11 weeks' gestation 9 months ago. Vital signs are within normal limits. Abdominal examination is unremarkable. On pelvic examination, there is old blood in the vaginal vault and at the closed cervical os. There are bilateral adnexal masses. Serum β-hCG concentration is 122,000 mIU/ml. Results from dilation and curettage show hydropic chorionic villi and proliferation of cytotrophoblasts and syncytiotrophoblasts. There are no embryonic parts. Vaginal ultrasound shows that both ovaries are enlarged and have multiple thin-walled, septated cysts with clear content. Which of the following is the most likely cause of the ovarian findings?