What is the most likely condition affecting the fetus of a 36-year-old woman with elevated alpha-fetoprotein levels?

Elevated alpha-fetoprotein (AFP) levels in the maternal serum during pregnancy are often associated with certain fetal conditions, particularly neural tube defects and abdominal wall defects. Anencephaly, a type of neural tube defect where the fetus is born without parts of the brain and skull, is one of the conditions that can lead to significantly increased levels of AFP. This is because the defect allows for the leakage of AFP into the maternal circulation.

In the case described, the 36-year-old woman's elevated AFP is most indicative of an issue related to the fetal development of the central nervous system or the abdominal wall, making anencephaly a likely contributor. Neural tube defects, including anencephaly, typically present with elevated AFP in maternal screening tests, guiding the healthcare team toward further evaluation and potential diagnostic imaging.

Other conditions such as fragile X syndrome and cystic fibrosis do not typically cause elevated AFP levels during pregnancy. Trisomy 21 (Down syndrome) can sometimes present with abnormal maternal serum screening results, but it is more often associated with low rather than elevated AFP. Thus, in this scenario, elevated AFP is most aligned with the risk of anencephaly, validating its selection as the most likely condition affecting the fetus.