Phelan-McDermid Syndrome 22q13.3

A 3-year-old boy with global developmental delay and profound expressive language impairment presents to the pediatric neurology clinic. Chromosomal microarray reveals a deletion at **A** (22q13.3). His parents report that he is essentially nonverbal despite appearing to understand simple commands. He also has neonatal-onset hypotonia, feeding difficulties, and a concerning history of not crying or withdrawing from painful stimuli. Which of the following best explains the neuropsychiatric phenotype in this child?