A 45-year-old man with a family history of early renal failure presents with hypertension (BP 160/95 mmHg) and bilateral flank pain. On examination, both kidneys are palpable and markedly enlarged. Imaging confirms autosomal dominant polycystic kidney disease (ADPKD). The structure marked **B** in the diagram shows markedly enlarged kidneys (>1 kg each). Which of the following genetic mutations is most likely responsible for this degree of renal enlargement and the patient's early presentation with hypertension?