A 21-year-old woman has just delivered a term infant. She has had only one visit to her obstetrician, and that was at about 6 weeks of pregnancy. She provides her laboratory results from that visit. The delivered infant is microcephalic, has cataracts, a heart murmur, and hepatosplenomegaly. Your further evaluation of the child demonstrates thrombocytopenia, mild hemolytic anemia, and, on the echocardiogram, patent ductus arteriosus and peripheral pulmonary artery stenosis. Which of the following maternal laboratory tests done at 6 weeks gestation is likely to explain the findings in this child?
a) Rubella IgG antibodies
b) Varicella IgM antibodies
c) Toxoplasma IgM antibodies
d) Cytomegalovirus (CMV) IgM antibodies