What test can be used to diagnose fetal CMV infection at 24 weeks gestation?

Amniocentesis is the correct test to diagnose fetal cytomegalovirus (CMV) infection at 24 weeks gestation. This procedure involves the extraction of amniotic fluid, which contains fetal cells, and allows for the direct testing of these cells for the presence of CMV.

Amniocentesis is particularly useful in identifying infections because it provides a sample that can be analyzed for viral DNA or RNA, making it a definitive diagnostic tool for congenital infections. The timing of 24 weeks is appropriate for this test as the procedure is typically performed between 15 and 20 weeks, but it can be done later in pregnancy if indicated.

While chorionic villus sampling can also diagnose genetic and some infectious conditions, it is usually performed earlier in pregnancy, around 10 to 13 weeks. Ultrasound can identify some features associated with fetal infections but does not provide a definitive diagnosis of CMV. Maternal blood tests can indicate the presence of antibodies against CMV, but this doesn't confirm an active infection in the fetus. Therefore, amniocentesis stands out as the most suitable option for direct diagnosis of fetal CMV infection at this stage of pregnancy.