Karyotype — Patau Syndrome Trisomy 13

A 2-day-old male neonate born to a 42-year-old primigravida presents with severe microcephaly, bilateral cleft lip and palate, postaxial polydactyly, and scalp defects with absent skin over the vertex. Karyotype analysis shows the abnormality marked **A** in the diagram. Which of the following is the most likely pathogenic mechanism underlying this karyotype abnormality in this patient?