A pregnant woman has positive IgG and IgM rubella tests. What positive test would confirm a diagnosis of congenital rubella syndrome in her baby after birth?

To confirm a diagnosis of congenital rubella syndrome in the newborn, the presence of positive IgM antibodies to rubella is critical. IgM is the first antibody produced in response to an infection and signifies a recent or acute infection. In the context of congenital infections, a positive IgM result in the neonate indicates that the infection was transmitted from the mother during pregnancy, thus confirming congenital rubella syndrome.

While IgG antibodies can provide information about past infections and can also be passed from mother to child, they do not definitively confirm active infection in the newborn. A positive IgG test could indicate maternal antibodies rather than an active infection. Therefore, the presence of IgM antibodies in the baby is much more indicative of congenital rubella.

The other options either pertain to IgG positivity, which does not confirm congenital rubella syndrome by itself, or suggest that no tests could confirm the diagnosis, which undermines the role of specific IgM testing in this context. Hence, the definitive indicator in this scenario is a positive rubella test for IgM.