In which of the following disorders would the maternal serum level of alpha-fetoprotein not be elevated?

Alpha-fetoprotein (AFP) is a protein produced by the fetal liver and is typically measured in maternal serum during pregnancy. Elevated levels of AFP can indicate various fetal conditions, while lower levels can suggest others.

In cases of neural tube defects and spina bifida, AFP levels are usually elevated because these conditions allow the protein to leak into the amniotic fluid and subsequently into the maternal serum. Therefore, in these disorders, you'd expect to find higher AFP levels.

Fetal distress, which can refer to abnormal fetal conditions leading to insufficient oxygen supply, might also be associated with elevated AFP levels depending on the underlying reason for the distress.

In contrast, Down syndrome, a chromosomal condition caused by the presence of an extra chromosome 21, is associated with a decrease in maternal serum AFP levels. This happens because the biochemical markers for Down syndrome tend to have altered patterns, often showing lower levels of AFP along with changes in other markers, leading to a more refined risk assessment for potential chromosomal abnormalities.

Consequently, the correct choice reflects the condition where the maternal serum level of alpha-fetoprotein would not be elevated, highlighting a significant aspect of prenatal screening for chromosomal abnormalities.