A 14-year-old boy is referred to pediatrics for evaluation of developmental delay and behavioral concerns. On examination, he is notably tall (height 188 cm, well above mid-parental height), has severe nodular cystic acne, and normal testicular size. Psychometric testing reveals an IQ of 92, with significant speech and language delay. A karyotype is performed showing the structure marked **A** in the diagram. Which of the following best explains the pathogenesis of this chromosomal abnormality?