Which finding on an ultrasound is indicative of trisomy 21?

C. Echogenic bowel is associated with trisomy 21 (Down syndrome). In prenatal screening, echogenic bowel can often be seen on an ultrasound and is considered a soft marker for various chromosomal abnormalities, including trisomy 21. This finding may signal the possibility of underlying issues, as echogenic bowel can also relate to other conditions or anomalies, but within the context of Down syndrome, it is significant.

Other indicators related to trisomy 21 include thick nuchal translucency, which is indeed another finding that can be seen on ultrasound in pregnancies at higher risk for Down syndrome. However, it is considered more specifically a risk factor rather than a definitive marker. A short femur is also a soft marker associated with various aneuploidies but is not as specific as echogenic bowel in identifying trisomy 21. Cloverleaf skull is not typically associated with trisomy 21; it is more characteristic of other syndromic conditions.