A term infant has initial Apgar scores of 8 and 10 at 1 and 5 minutes. On auscultation of the chest, a heart murmur is audible. There is hepatosplenomegaly. Cataracts of the crystalline lens are noted. The infant is at the 30th percentile for height and weight. Echocardiography shows a patent ductus arteriosus. Which of the following events is the most likely risk factor for the findings in this infant?

Rubella infection in the first trimester, when organogenesis is occurring (3 to 9 weeks' gestation), can lead to embryopathy with cardiac, ocular, skin, central nervous system, and hepatic abnormalities. Rubella is rare with routine vaccinations. Folate deficiency is most likely to be associated with neural tube defects. Dispermy is rare and can lead to triploidy, a condition that rarely results in a live birth. Erythroblastosis fetalis leads to fetal anemia with congestive heart failure and hydrops, but not to malformations. In the past, thalidomide use was a cause of malformations (almost invariably prominent limb deformities). Nondisjunctional events during meiosis in the maternal ova account for trisomies and monosomies, many of which have associated cardiac defects, including ventricular septal defect (e.g., trisomy 21). This mechanism is unlikely in paternal sperm, however, which are constantly being produced in large numbers throughout life.