**Core Concept**
Diabetic nephropathy, also known as diabetic kidney disease, is a common complication of diabetes mellitus characterized by progressive renal fibrosis and loss of kidney function. It is primarily caused by prolonged hyperglycemia, which leads to mesangial expansion, basement membrane thickening, and glomerular sclerosis.

**Why the Correct Answer is Right**
The characteristic finding in diabetic nephropathy is the accumulation of extracellular matrix proteins, particularly collagen IV, in the mesangial region of the glomerulus, leading to mesangial expansion and thickening of the glomerular basement membrane. This process disrupts the normal glomerular filtration barrier, impairing kidney function. The thickening of the glomerular basement membrane is a hallmark of diabetic nephropathy and can be detected by electron microscopy or immunofluorescence.

**Why Each Wrong Option is Incorrect**
* **Option A:** While proteinuria is a common feature of diabetic nephropathy, it is not the characteristic finding. Proteinuria is a symptom of kidney damage, but it is not the underlying cause or hallmark of the disease.
* **Option B:** Glomerulonephritis is a different type of kidney disease characterized by inflammation of the glomeruli, which is not a characteristic feature of diabetic nephropathy.
* **Option C:** Nephrotic syndrome is a condition characterized by severe proteinuria, hypoalbuminemia, and edema, which can be a complication of diabetic nephropathy, but it is not the characteristic finding.

**Clinical Pearl / High-Yield Fact**
In diabetic nephropathy, the thickening of the glomerular basement membrane is directly proportional to the duration of diabetes, making it a useful marker for monitoring disease progression.

**Correct Answer:** C. Glomerular basement membrane thickening