A 52-year-old man with type 2 diabetes mellitus presents with persistent hyperglycemia despite oral antidiabetic therapy. His fasting blood glucose is 180 mg/dL and HbA1c is 9.2%. However, his blood glucose improves significantly after 30 minutes of brisk walking. The structure marked **D** in the diagram represents GLUT4 vesicle translocation to the plasma membrane in skeletal muscle. Which of the following best explains why exercise-induced glucose uptake occurs in this patient despite impaired insulin signaling?