On physical examination, a 1-day-old child in the newborn nursery has a noisy, holosystolic murmur. The baby was delivered at 39 weeks to a 36-year-old mom who chose not to get prenatal screening. According to a review of medical data, there is no family history of genetic or chromosomal problems. The infant's vital signs are within normal limits for his age. The remainder of the physical exam reveals a flat face profile, a projecting tongue, and tiny ears. Which of the following events is most likely to have occurred prior to conception?

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Eductional Objective: The most prevalent cause of Down syndrome is maternal meiotic nondisjunction, a procedure in which the fetus obtains three complete copies of chromosome 21. Characteristic traits include dysmorphic features (such as a flat facial profile, projecting tongue, tiny ears, and upslanting palpebral fissures) and heart anomalies (such as endocardial cushion deficiencies).

On Physical Examination, a 1-Day-Old Child in the Newborn Nursery