Marker of neural tube defects is:
**Core Concept**
Neural tube defects (NTDs) are congenital anomalies that occur during embryonic development, specifically between the third and fourth weeks of gestation. These defects result from the incomplete closure of the neural tube, leading to various abnormalities in brain and spine formation.
**Why the Correct Answer is Right**
The marker of choice for neural tube defects is Maternal Serum Alpha-Fetoprotein (MSAFP). Alpha-fetoprotein (AFP) is a protein produced by the yolk sac and fetal liver during pregnancy. Elevated levels of MSAFP are associated with an increased risk of NTDs, as the open neural tube allows AFP to leak into the maternal serum. The detection of elevated MSAFP levels prompts further investigation, such as ultrasound or amniocentesis, to confirm the diagnosis.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as there is no well-established marker for neural tube defects among the common prenatal screening tests.
* **Option B:** This option is incorrect as Human Chorionic Gonadotropin (hCG) is a marker for Down syndrome, not neural tube defects.
* **Option C:** This option is incorrect as Inhibin-A is a marker for Down syndrome and trisomy 18, not neural tube defects.
**Clinical Pearl / High-Yield Fact**
Remember that an elevated MSAFP level can be a marker for neural tube defects, but it can also be associated with other conditions, such as multiple gestations or fetal demise. Always consider the clinical context and perform further testing to confirm the diagnosis.
**Correct Answer:** D.