**Core Concept**
Increased Maternal Serum Alpha-Fetoprotein (MSAFP) during pregnancy is a marker for potential fetal abnormalities, particularly neural tube defects (NTDs) and abdominal wall defects. MSAFP is a protein produced by the yolk sac and fetal liver during embryonic development.

**Why the Correct Answer is Right**
Amniocentesis is a diagnostic procedure that involves withdrawing a sample of amniotic fluid from the uterus to assess fetal health. In cases of increased MSAFP, amniocentesis can help identify the underlying cause, such as an open neural tube defect or abdominal wall defect. The amniotic fluid analysis can also provide information on fetal chromosomal abnormalities and other genetic disorders. Amniocentesis is typically performed between 15 and 20 weeks of gestation.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is a valuable tool for assessing fetal anatomy and growth, but it may not provide a definitive diagnosis for increased MSAFP. While ultrasound can identify some fetal abnormalities, it cannot rule out the presence of a neural tube defect or other genetic disorders.
**Option B:** Chorionic villus sampling (CVS) is a prenatal diagnostic test that involves removing a small sample of placental tissue. While CVS can diagnose chromosomal abnormalities and some genetic disorders, it is typically performed earlier in pregnancy (between 10 and 13 weeks) and may not be indicated in cases of increased MSAFP.
**Option C:** Non-stress test (NST) is a fetal monitoring test that assesses fetal well-being by measuring fetal heart rate in response to fetal movement. While an NST can provide information on fetal well-being, it is not a diagnostic test for increased MSAFP.

**Clinical Pearl / High-Yield Fact**
Increased MSAFP is often associated with a higher risk of neural tube defects, including spina bifida and anencephaly. A level of MSAFP above 2.5 multiples of the median (MoM) is considered elevated and warrants further investigation.

**Correct Answer:** C.