**Core Concept**
An open neural tube defect (NTD) such as spina bifida or anencephaly can be detected in maternal serum through specific biochemical markers. These markers are elevated due to the abnormal development of the neural tube during embryogenesis.

**Why the Correct Answer is Right**
Maternal serum alpha-fetoprotein (MSAFP) is a protein produced by the yolk sac and fetal liver. In an open NTD, the neural tube is exposed, leading to the leakage of MSAFP into the maternal circulation, causing elevated levels. This is due to the impaired closure of the neural tube, resulting in the release of MSAFP into the amniotic fluid and subsequently into the maternal bloodstream.

**Why Each Wrong Option is Incorrect**
* **Option A:** Human chorionic gonadotropin (hCG) is a hormone produced by the placenta and is not directly related to neural tube defects.
* **Option B:** Estriol (E3) is an estrogen metabolite produced by the fetal liver and placenta, but its levels are not specifically indicative of open NTDs.
* **Option C:** Inhibin-A is a protein produced by the fetal liver and ovaries, but its levels are not directly related to neural tube defects.

**Clinical Pearl / High-Yield Fact**
Elevated MSAFP levels in maternal serum can also be seen in other conditions such as multiple pregnancy, fetal death, or placental abruption, making it essential to interpret these results in conjunction with other diagnostic findings.

**Correct Answer:** A. Alpha-fetoprotein