A 23-year-old woman consults an obstetrician because of the onset of vaginal bleeding in what she considers to be the fifth month of pregnancy; however, examination reveals the uterus to be enlarged to the size of a 7-month pregnancy. Intravaginal ultrasound fails to detect a fetal heabeat and instead shows a "snowstorm pattern." HCG is markedly elevated. These findings are strongly suggestive of