**Core Concept**
Diabetic nephropathy, also known as diabetic kidney disease, is a complication of diabetes mellitus that affects the kidneys' ability to filter waste and excess fluids from the blood. The earliest finding in diabetic nephropathy involves changes in the kidneys' glomeruli, which are the tiny blood vessels responsible for filtering waste and excess fluids.

**Why the Correct Answer is Right**
The earliest finding in diabetic nephropathy is the thickening of the glomerular basement membrane, which is a consequence of increased synthesis of collagen and other extracellular matrix proteins. This thickening leads to a reduction in glomerular filtration rate and is a hallmark of early diabetic nephropathy. The increased synthesis of extracellular matrix proteins is mediated by the activation of the renin-angiotensin-aldosterone system and the subsequent release of transforming growth factor-beta (TGF-β).

**Why Each Wrong Option is Incorrect**
**Option A:** Increased proteinuria is a later finding in diabetic nephropathy, typically seen after the thickening of the glomerular basement membrane.
**Option B:** Nephrotic syndrome is a later complication of diabetic nephropathy, characterized by severe proteinuria, hypoalbuminemia, and edema.
**Option C:** Renal calculi are not directly related to diabetic nephropathy and are a separate entity.

**Clinical Pearl / High-Yield Fact**
Remember that the earliest finding in diabetic nephropathy is the thickening of the glomerular basement membrane, which is often asymptomatic and may not be detected until it is advanced.

**Correct Answer:** C. Thickening of the glomerular basement membrane.