Fragile X Syndrome (FMR1, X-linked)

A 6-year-old boy presents with global developmental delay, autistic features (poor eye contact, hand-flapping), and hyperactivity. On examination, he has a long narrow face, prominent forehead, large protuberant ears, and hyperextensible joints. His maternal uncle has fragile X-associated tremor/ataxia syndrome. Karyotype shows 46,XY with a folate-sensitive fragile site. Southern blot confirms a full mutation at the structure marked **A** in the diagram. Which of the following best describes the molecular consequence of this mutation?