**Core Concept**
Nephrotic syndrome is a clinical disorder characterized by massive proteinuria, hypoalbuminemia, hyperlipidemia, and edema. The primary pathophysiological mechanism involves damage to the glomerular filtration barrier, leading to excessive loss of albumin and other proteins in the urine.

**Why the Correct Answer is Right**
The characteristic feature of nephrotic syndrome in children is the presence of minimal change disease (MCD), which accounts for approximately 80-90% of cases. MCD is characterized by normal glomeruli on light microscopy, but with podocyte effacement and foot process fusion on electron microscopy. This leads to the loss of negative charge on the glomerular basement membrane, resulting in increased permeability and excessive proteinuria.

**Why Each Wrong Option is Incorrect**
**Option A:** Focal segmental glomerulosclerosis (FSGS) is a cause of nephrotic syndrome in children, but it is not the most common form. FSGS is characterized by sclerosis of some glomeruli and is often associated with other underlying conditions.
**Option B:** Membranous nephropathy is a cause of nephrotic syndrome in adults, but it is rare in children. It is characterized by thickening of the glomerular basement membrane due to immune complex deposition.
**Option C:** Membranoproliferative glomerulonephritis is a cause of nephritic syndrome, not nephrotic syndrome. It is characterized by thickening of the glomerular basement membrane and an increase in mesangial cells and matrix.

**Clinical Pearl / High-Yield Fact**
Minimal change disease is often associated with a good response to corticosteroid therapy, and the majority of children with MCD achieve complete remission with treatment.

**Correct Answer: A. Minimal change disease.**