Answer: A

Reasoning: Prenatal diagnosis of Down syndrome is achieved with biochemical serum screening (e.g., alpha-fetoprotein), or invasive karyotyping procedures such as chorionic-villus sampling, mid-trimester amniocentesis, or fetal blood sampling by cordocentesis [Herlicoviez, 1990, PMID 2206291; Newberger, 2000, PMID 10969860]. Ultrasonographic estimation of amniotic-fluid volume does not identify the chromosomal abnormality and is therefore not part of Down-syndrome diagnostic protocols.