A 52-year-old man with a 10-year history of type 2 diabetes mellitus presents to the nephrology clinic for evaluation of progressive renal disease. His serum creatinine is 1.8 mg/dL (baseline 0.9 mg/dL 2 years ago), and 24-hour urine protein is 2.5 g/day. His blood pressure is 148/92 mmHg. On examination, he has mild peripheral edema. Laboratory investigations show: serum urea 52 mg/dL, serum sodium 138 mEq/L, serum potassium 5.2 mEq/L. His 24-hour urine creatinine excretion is 1.2 g/day. Using the Cockcroft-Gault equation, his estimated GFR is approximately 45 mL/min/1.73 m². What is the primary mechanism responsible for the decline in his GFR in diabetic nephropathy?