A 24-year-old woman has a marked increase in the size of her uterus between 26 and 30 weeks' gestation. Ultrasonography shows a marked increase in the amount of amniotic fluid; the fetus is normal in size for gestational age. Which of the following is the most likely diagnosis?
a. Congenital heart disease
b. Duodenal atresia
c. Erythroblastosis fetalis
d. Horseshoe kidney
e. Neural tube defect