A 28-week-old fetus is referred for detailed ultrasound following abnormal quad screening showing low AFP, low estriol, low hCG, and low inhibin A. Fetal biometry reveals severe intrauterine growth restriction despite appropriate gestational age, microcephaly, and characteristic clenched fists with overlapping fingers. Karyotype analysis confirms the presence of three copies of chromosome 18, as shown by the structure marked **A** in the diagram. Which of the following best describes the embryologic basis for the aneuploidy in this case?