A 32-year-old woman from Mumbai presents to the genetics clinic with a family history of progressive muscle weakness. Her father and paternal uncle both developed symptoms in their 40s and are now wheelchair-bound. Genetic testing reveals a mutation in the DMD gene (dystrophin gene) affecting the splice donor site of exon 45. The mutation does not completely prevent splicing but causes aberrant splicing with inclusion of a cryptic exon. Which of the following best explains the molecular consequence of this mutation and the resulting clinical phenotype?