**Core Concept**
Diabetic nephropathy is a microvascular complication of diabetes mellitus characterized by damage to the glomeruli, leading to proteinuria and progressive renal failure. The histological lesions in diabetic nephropathy are a result of hyperglycemia-induced changes in the glomerular basement membrane and mesangial cells.

**Why the Correct Answer is Right**
The most specific histological lesion in diabetic nephropathy is the **mesangial expansion**. This is due to the accumulation of mesangial matrix material, including collagen and fibronectin, which is induced by the activation of transforming growth factor-beta (TGF-β) and other growth factors. The mesangial expansion leads to thickening of the glomerular basement membrane and narrowing of the glomerular capillaries, resulting in decreased glomerular filtration rate and proteinuria.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glomerular hypertrophy is a feature of diabetic nephropathy, but it is not the most specific histological lesion.
* **Option B:** Tubulointerstitial fibrosis is a common feature of various kidney diseases, including diabetic nephropathy, but it is not the most specific lesion for diabetic nephropathy.
* **Option C:** Kimmelstiel-Wilson nodules are a characteristic feature of diabetic nephropathy, but they are not the most specific histological lesion.

**Clinical Pearl / High-Yield Fact**
The mesangial expansion in diabetic nephropathy is reversible with tight glycemic control, making early detection and management of diabetes crucial in preventing the progression of the disease.

**Correct Answer: C. Kimmelstiel-Wilson nodules. Kimmelstiel-Wilson nodules are a characteristic feature of diabetic nephropathy.**