**Core Concept**
Diabetic nephropathy is a leading cause of chronic kidney disease and end-stage renal disease worldwide, often presenting with proteinuria, hypertension, and renal insufficiency. The pathophysiology involves hyperglycemia-induced damage to the glomerular basement membrane and mesangial expansion.

**Why the Correct Answer is Right**
The patient's long-standing type 2 diabetes, proteinuria, and hypertension are classic findings for diabetic nephropathy. The absence of hematuria and normal-sized kidneys on ultrasound further support this diagnosis. Diabetic nephropathy typically progresses to renal insufficiency over years, which is consistent with the patient's symptoms of fatigue and pedal edema.

**Why Each Wrong Option is Incorrect**
**Option A:** Focal segmental glomerulosclerosis (FSGS) is a cause of nephrotic syndrome, but it is less likely in this patient with long-standing diabetes. FSGS often presents with more severe proteinuria and a more rapid decline in kidney function.
**Option B:** Membranous nephropathy is a cause of nephrotic syndrome, but it is more common in adults without a history of diabetes. This patient's long-standing diabetes makes diabetic nephropathy a more likely diagnosis.
**Option C:** Rapidly progressive glomerulonephritis (RPGN) is a cause of acute kidney injury, but it typically presents with hematuria, proteinuria, and a rapid decline in kidney function. This patient's presentation is more consistent with chronic kidney disease.
**Option D:** Minimal change disease is a cause of nephrotic syndrome, but it is more common in children and often presents with more severe proteinuria and a more rapid response to corticosteroid therapy.

**Clinical Pearl / High-Yield Fact**
Diabetic nephropathy is the leading cause of end-stage renal disease worldwide, and early detection and management are crucial to prevent progression. Regular monitoring of urine protein-to-creatinine ratio and serum creatinine levels is essential in patients with diabetes.

**Correct Answer:** C. Diabetic nephropathy.