A 52-year-old man with type 2 diabetes presents with progressive proteinuria and declining GFR. His serum creatinine is 2.8 mg/dL and urine protein is 3.5 g/day. Which of the following is the most common cause of reduced GFR in this patient?

Explanation

## Diabetic Nephropathy: Pathophysiology of GFR Decline ### Stages of Diabetic Kidney Disease | Stage | GFR (mL/min/1.73m²) | Pathology | Proteinuria | |-------|---------------------|-----------|-------------| | **Hyperfiltration** | >120 | Glomerular enlargement | Absent | | **Silent** | 90–120 | Basement membrane thickening | Microalbuminuria | | **Overt** | 60–90 | Nodular glomerulosclerosis | Macroproteinuria | | **Progressive CKD** | <60 | Advanced sclerosis, tubular atrophy | Heavy proteinuria | ## Most Common Cause of GFR Reduction **Key Point:** Glomerular basement membrane (GBM) thickening with nodular glomerulosclerosis (Kimmelstiel-Wilson lesions) is the pathognomonic and most common cause of GFR decline in diabetic nephropathy. **High-Yield:** Nodular glomerulosclerosis is the hallmark of diabetic kidney disease and is present in ~30% of patients with type 2 diabetes and proteinuria. It represents irreversible glomerular damage. ### Mechanism of GFR Decline in Diabetic Nephropathy 1. **Initial phase (years 0–5):** Afferent arteriole vasodilation → hyperfiltration → GFR initially elevated 2. **Transition phase (years 5–15):** GBM thickening, mesangial expansion, early nodular lesions 3. **Progressive phase (>15 years):** Nodular glomerulosclerosis → loss of filtration surface area → GFR decline 4. **End-stage:** Global sclerosis, tubular atrophy, interstitial fibrosis **Clinical Pearl:** The presence of nodular glomerulosclerosis with GBM thickening >400 nm (normal <300 nm) on electron microscopy confirms diabetic nephropathy and is associated with rapid GFR decline. **Mnemonic:** **KW-DM** — Kimmelstiel-Wilson lesions, Diabetic Membrane thickening ### Why GFR Declines Despite Initial Hyperfiltration - Progressive glomerular sclerosis reduces the effective filtration surface area - GBM thickening impairs size-selective filtration - Nodular lesions obstruct capillary lumens - Tubular atrophy and interstitial fibrosis reduce functional nephron mass ```mermaid flowchart TD A[Type 2 Diabetes]:::outcome --> B[Hyperglycemia + Hypertension]:::outcome B --> C[Afferent vasodilation<br/>Efferent vasoconstriction]:::action C --> D[Hyperfiltration<br/>GFR > 120]:::outcome D --> E[Glomerular injury<br/>GBM thickening]:::urgent E --> F[Mesangial expansion<br/>Nodular sclerosis]:::urgent F --> G[Loss of filtration surface]:::urgent G --> H[GFR decline<br/>Proteinuria]:::outcome ``` [cite:Harrison 21e Ch 297]