**Core Concept**
Maternal alpha-fetoprotein (AFP) levels are a crucial marker in prenatal screening for fetal abnormalities. Elevated levels of AFP in maternal serum are associated with an increased risk of certain fetal conditions.

**Why the Correct Answer is Right**
Elevated maternal AFP levels are commonly seen in cases of neural tube defects (NTDs), such as spina bifida or anencephaly. This occurs due to the disruption of the normal blood-brain barrier, leading to the leakage of AFP from the fetal brain into the amniotic fluid and subsequently into the maternal circulation. The enzyme responsible for the breakdown of AFP, acetylcholinesterase, is also elevated in cases of NTDs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because AFP is not typically elevated in cases of chromosomal abnormalities such as Down syndrome.
* **Option B:** This option is incorrect because AFP is not directly related to fetal sex determination.
* **Option C:** This option is incorrect because AFP is not a marker for fetal anemia.

**Clinical Pearl / High-Yield Fact**
Elevated maternal AFP levels can also be seen in cases of multiple gestations, fetal demise, or maternal liver disease, highlighting the importance of careful interpretation of AFP results in the context of the patient's overall clinical picture.

**Correct Answer: A. Neural tube defects.**