Cystic Fibrosis — Autosomal Recessive with Consanguinity

A 3-year-old boy of South Indian ancestry presents with recurrent bronchiectasis, steatorrhea, and failure to thrive. His parents are first cousins and phenotypically healthy; his younger sister has similar symptoms. Sweat chloride is 82 mmol/L on two occasions (positive threshold ≥60 mmol/L). The pedigree shows a double horizontal line between the unaffected parents with two of four children affected and no affected individuals in prior generations. The structure marked **B** in the diagram represents the gene responsible for this condition. Which of the following best describes the molecular defect at this locus?