A 45-year-old man presents with progressive distal weakness, facial wasting, and difficulty releasing his grip after shaking hands. His 12-year-old son is found to have the same condition but with earlier onset and more severe myotonia. Genetic testing confirms myotonic dystrophy type 1. The pedigree pattern shown in the diagram demonstrates the phenomenon marked **A**, which is characterized by progressively earlier age of onset and increased disease severity across successive generations. This phenomenon in myotonic dystrophy type 1 is caused by which of the following molecular mechanisms?