Achondroplasia FGFR3 Autosomal Dominant

A 3-month-old boy presents with progressive developmental delay and episodes of central apnoea during sleep. Examination reveals macrocephaly with frontal bossing, midface hypoplasia, and rhizomelic shortening of the limbs. The infant's father is 42 years old and has normal stature. Molecular testing confirms a de novo heterozygous G380R mutation in FGFR3 on chromosome 4p16.3. The condition marked **A** in the diagram is suspected. Which of the following best explains the molecular basis of the growth restriction in this infant?