Fragile X Manifesting Carrier

A 7-year-old girl presents with mild intellectual disability (IQ 72), learning difficulties, and anxiety. Her father has moderate intellectual disability with characteristic long narrow face, large ears, and macroorchidism. Genetic testing reveals a full mutation (>200 CGG repeats) in the FMR1 gene on Xq27.3 with hypermethylation. The inheritance pattern marked **B** in the diagram — X-linked recessive with manifesting female carrier due to skewed Lyonization — best explains her clinical presentation. Which of the following BEST describes the molecular mechanism underlying her intellectual disability despite being heterozygous for the FMR1 mutation?