A 3-week-old male infant presents with seizures and tetany. Serum calcium is 6.2 mg/dL (normal 8.5–10.5). On examination, he has a long face, low-set ears, micrognathia, and a submucous cleft palate. Echocardiography reveals tetralogy of Fallot. The karyotype shows a heterozygous microdeletion on chromosome 22q11.2. The clinical diagnosis marked **D** in the diagram is most consistent with which of the following management priorities?