IgM anti-rubella antibodies in the serum from an infant with a rash suggests which diagnosis?

• A. measles
• B. congenital rubella infection
• C. German measles
• D. passive transfer of maternal antibodies to the infant's serum

Answer: (B)

IgM antibodies are produced by the newborn in response to an infection and do not cross the placenta. Seeing rubella-specific IgM in an infant’s serum means the infant itself has been infected and mounted an immune response, i.e., congenital rubella infection. If the signal were from maternal antibodies, you’d expect IgG transferred from mother, not IgM. Measles would involve anti-measles antibodies, and while “German measles” is another name for rubella, the presence of IgM against rubella in the infant specifically points to congenital rubella infection rather than passive transfer of maternal antibodies.