During her eighth month of pregnancy, a 29-year-old woman is noted to have hydramnios. Further testing shows anencephalous. In this case hydramnios is caused by which of the following?

This abnormality is relatively common and occurs in 1 of 1000 pregnancies. It occurs four times more commonly in whites than blacks and four times more commonly in female fetuses than in male fetuses. The abnormality can be identified on an x-ray because the vault of the skull is absent. Anencephalous is caused by failure of the cephalic part of the neural tube to close off.