A 7-month-old boy presents with recurrent bacterial infections including otitis media and pneumonia caused by *Streptococcus pneumoniae*. Laboratory studies show profoundly reduced levels of all immunoglobulin classes (IgG, IgM, IgA) and absent tonsils on examination. Flow cytometry reveals severely reduced B-cell numbers with arrest at the pre-B-cell stage in the bone marrow. The structure marked **C** in the diagram represents the genetic defect responsible for this clinical presentation. Which of the following best describes the molecular basis of the condition marked **C**?