**Core Concept**
Neural tube defects (NTDs) are congenital anomalies resulting from the incomplete closure of the neural tube during embryonic development. The neural tube gives rise to the brain and spinal cord. Early detection of NTDs is crucial for timely intervention and management.

**Why the Correct Answer is Right**
Alpha-fetoprotein (AFP) is a specific marker used in prenatal screening for neural tube defects. Elevated levels of AFP in maternal serum and amniotic fluid are associated with an increased risk of NTDs. The mechanism involves the disruption of the neural tube, leading to the leakage of AFP into the amniotic fluid and maternal circulation. AFP is produced by the yolk sac and fetal liver during embryonic development, but its levels decrease after birth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because human chorionic gonadotropin (hCG) is a different marker used in prenatal screening, primarily for detecting Down syndrome and other chromosomal abnormalities. While hCG levels can be affected in pregnancies with NTDs, it is not a specific marker for neural tube defects.

**Option B:** This option is incorrect because estriol (E3) is another hormone used in prenatal screening, primarily for detecting Down syndrome and other chromosomal abnormalities. Like hCG, estriol levels can be affected in pregnancies with NTDs, but it is not a specific marker for neural tube defects.

**Option C:** This option is incorrect because ultrasound is a diagnostic tool used to detect neural tube defects, but it is not a specific marker. Ultrasound can visualize the fetal anatomy, including the neural tube, and detect abnormalities such as spina bifida.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women at high risk for NTDs undergo amniocentesis or chorionic villus sampling (CVS) for definitive diagnosis.

**Correct Answer: D. Alpha-fetoprotein (AFP)**