DiGeorge syndrome 22q11.2 microdeletion

A 6-week-old male infant presents with seizures (serum calcium 6.2 mg/dL, ionized 0.8 mmol/L) and a cardiac murmur consistent with tetralogy of Fallot. Dysmorphic features include micrognathia, hypertelorism, low-set posteriorly rotated ears, and a bifid uvula. Chest X-ray shows an absent thymic shadow, and flow cytometry reveals T-cell lymphopenia (CD3+ 800/μL, CD4+ 400/μL). FISH karyotype using the TUPLE1 probe shows the abnormality marked **C** in the diagram. Which of the following best explains the pathophysiology of this patient's constellation of findings?