**Core Concept**
Diabetic nephropathy, a complication of diabetes mellitus, is characterized by progressive renal damage and dysfunction. The underlying pathophysiological mechanism involves hyperglycemia-induced renal fibrosis, inflammation, and oxidative stress.

**Why the Correct Answer is Right**
The most characteristic feature of diabetic nephropathy is the **mesangial expansion** and deposition of **excess matrix** in the glomeruli, leading to thickening of the glomerular basement membrane. This is due to the activation of the **transforming growth factor-beta (TGF-β) pathway**, which promotes the expression of collagen and other matrix proteins. The resulting renal fibrosis impairs glomerular filtration and ultimately leads to nephrotic syndrome and end-stage renal disease.

**Why Each Wrong Option is Incorrect**
**Option A:** While hypertension is a common comorbidity in diabetic nephropathy, it is not the most characteristic feature of the disease.
**Option B:** Proteinuria is a hallmark of diabetic nephropathy, but it is not the most specific or characteristic feature.
**Option C:** Hyperfiltration is an early change in diabetic nephropathy, but it is not the most characteristic feature of the disease.

**Clinical Pearl / High-Yield Fact**
Diabetic nephropathy is the leading cause of chronic kidney disease and end-stage renal disease worldwide.

**Correct Answer:** B. Proteinuria