A 3-week-old neonate presents with cyanotic heart disease (tetralogy of Fallot), seizures due to hypocalcemia, and recurrent candidal infections. FISH analysis is performed on metaphase chromosomes using a TUPLE1 probe (red signal at 22q11.2) and a control probe at 22q13 (green signal). The structure marked **A** in the diagram shows absent red probe signal on one chromosome 22, while the homologous chromosome 22 shows both red and green signals. Which of the following best explains the pathogenesis of this patient's constellation of clinical features?