A 12-year-old boy presents with generalized dry, scaly skin since infancy that worsens in winter. He has a personal history of atopic dermatitis, allergic rhinitis, and asthma, and his father has a similar skin condition. On examination, the fine white polygonal scales marked **A** are prominent on extensor surfaces of the lower extremities and trunk, with characteristic sparing of flexural creases. Hyperlinear palms and keratosis pilaris are noted. Which of the following genetic defects best explains the pathophysiology underlying this clinical presentation?