A 52-year-old woman with a 10-year history of type 2 diabetes mellitus presents with progressive renal dysfunction. Her serum creatinine is 2.8 mg/dL (eGFR 22 mL/min/1.73 m²), and urinalysis shows 3+ proteinuria with no hematuria. Kidney ultrasound shows normal-sized kidneys without obstruction. A renal biopsy reveals nodular glomerulosclerosis with basement membrane thickening. Which segment of the nephron is PRIMARILY affected in this patient, and what is the functional consequence?

## Diabetic Nephropathy Pathology **Key Point:** Diabetic nephropathy is a **glomerular disease** characterized by progressive damage to the filtration barrier, resulting in proteinuria and declining GFR. The hallmark histologic finding is **nodular glomerulosclerosis (Kimmelstiel-Wilson lesions)**. ## Structure and Function of the Glomerular Filtration Barrier The filtration barrier comprises three layers: | Layer | Composition | Function in DM | |-------|-------------|----------------| | **Endothelial cells** | Fenestrated capillary endothelium | Damaged by hyperglycemia; loss of glycocalyx | | **Basement membrane** | Type IV collagen, laminin, proteoglycans | **Thickened** in DM; reduced charge selectivity | | **Podocytes** | Visceral epithelial cells with foot processes | **Effaced** in DM; loss of slit diaphragm proteins (nephrin, podocin) | **High-Yield:** In diabetic nephropathy: - Hyperglycemia → advanced glycation end products (AGEs) → glomerular basement membrane thickening - Glomerular hypertension → podocyte injury and foot process effacement - Loss of charge and size selectivity → **proteinuria** (initially albumin, then larger proteins) - Progressive glomerulosclerosis → reduction in functional glomeruli → **declining GFR** ## Why This Patient Has Proteinuria and Renal Failure 1. **Nodular glomerulosclerosis** = replacement of glomerular capillaries with hyaline material (PAS-positive) 2. **Basement membrane thickening** = impaired filtration selectivity 3. **Podocyte loss** = inability to maintain filtration barrier integrity 4. **Result:** Proteinuria (3+) + declining GFR (eGFR 22) + preserved kidney size (no obstruction) **Clinical Pearl:** The **glomerulus is the only site where filtration occurs**. Damage here directly impairs GFR. Tubular dysfunction (proximal, loop, distal) causes electrolyte and acid-base disturbances but does NOT reduce GFR or cause proteinuria unless glomerular damage is present.