**Core Concept:** Neural tube defects (NTDs) are a group of congenital malformations originating from the failure of the neural tube to close during embryonic development. The most common NTDs are anencephaly, spina bifida, and encephalocele. Serum screening tests like combined test and quadruple test are used to identify pregnant women at high risk for having a baby with NTDs. The combined test involves measuring the levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and estriol (E3), along with maternal age, to calculate a risk score.

**Why the Correct Answer is Right:** The correct answer is D, which refers to the measurement of alpha-fetoprotein (AFP) levels in the mother's serum during pregnancy. AFP is a protein produced by the liver of the developing fetus and is transported into the mother's bloodstream. In the context of NTD screening, a high AFP level indicates a higher risk of NTDs due to increased fetal leakage or leakage of AFP into the maternal circulation.

**Why Each Wrong Option is Incorrect:**

A. Human chorionic gonadotropin (hCG): This is incorrect because it is part of the combined test, not specifically related to NTD screening.
B. Estriol (E3): This is incorrect because E3 is part of the combined test, not relevant to NTD screening on its own.
C. Maternal age: This is incorrect because maternal age is a factor in calculating the risk score of the combined test, not directly related to NTD screening.

**Clinical Pearl:**

Although the combined test is a useful tool for NTD screening, it is essential to remember that the primary screening test for NTDs is the quadruple test. The quadruple test evaluates the levels of AFP, hCG, E3, and Inhibin-A. If the risk score indicates a high risk of NTDs, further evaluation is required, such as amniocentesis to analyze the amniotic fluid for AFP, hCG, and Inhibin-A levels, as well as chromosomal analysis.