Which of the following ultrasound findings is considered classic for trisomy 21?

Echogenic bowel is a classic ultrasound finding associated with trisomy 21, also known as Down syndrome. This finding refers to the appearance of the fetal bowel on an ultrasound that has increased echogenicity compared to the surrounding structures. It can be an indicator of several conditions, and its presence in the context of other markers raises suspicion for chromosomal abnormalities, particularly trisomy 21.

The significance of echogenic bowel lies in its association with additional risk factors that contribute to the overall evaluation of potential genetic concerns. While it may not be exclusive to Down syndrome and can occur in other conditions, it is one of the recognized ultrasound markers that can lead to further genetic testing or evaluation.

Other options present findings that may be related to different conditions or are not as strongly associated with trisomy 21 specifically. For example, clubbed feet and midline craniofacial abnormalities are more general findings and are not definitive indicators. Posterior fossa abnormalities can also be seen in other types of congenital malformations and are not unique to trisomy 21. Hence, the identification of echogenic bowel stands out as a classic marker for this particular chromosomal condition, supporting its selection in this context.