What is the significance of identifying 47,XX,+21 in prenatal cytogenetics?

Identifying a karyotype of 47,XX,+21 in prenatal cytogenetics is significant because it indicates the presence of an extra chromosome 21, which is characteristic of Down syndrome, also known as Trisomy 21. This chromosomal abnormality results in a total of 47 chromosomes instead of the typical 46, with the extra chromosome leading to the phenotypic features and developmental challenges associated with the condition.

Down syndrome is typically associated with physical characteristics such as distinct facial features, hypotonia, and an increased risk for certain medical conditions. Additionally, early identification of Down syndrome can allow for better prenatal care, planning, and support for the parents, which is crucial for the management of potential health issues that may arise after birth.

The other options do not represent the significance of this specific karyotype. A normal chromosomal count would show 46 chromosomes, and Turner syndrome is indicated by a missing sex chromosome (45,X) rather than an extra chromosome 21. While identifying chromosomal abnormalities can indicate the possibility of genetic disorders, the specific karyotype of 47,XX,+21 directly correlates with Down syndrome, making that answer the most relevant in this context.