Increased levels of a-fetoprotein can indicate which neural tube defect?

Alpha-fetoprotein (AFP) is a protein that is produced primarily by the fetal liver and is present in the mother's blood during pregnancy. Elevated levels of AFP can be associated with various fetal abnormalities, particularly neural tube defects. Anencephaly, which is characterized by the absence of major portions of the brain, skull, and scalp, is one of the conditions strongly correlated with increased AFP levels.

In cases of anencephaly, the neural tube fails to close properly during early development, which leads to significant structural abnormalities. The exposed neural tissue is thought to leak AFP into the amniotic fluid and subsequently into the maternal circulation, resulting in elevated levels detectable through maternal blood tests.

While other conditions such as encephalocele can also result in elevated AFP levels due to disruptions in neural tube development, anencephaly is particularly noted for its strong association with significantly increased AFP levels in maternal serum. Thus, the elevated levels of AFP in the context of a neural tube defect are most specifically indicative of anencephaly, making it the correct choice in this scenario.