**Core Concept**
Nephrotic syndrome in the pediatric age group is a clinical condition characterized by massive proteinuria, hypoalbuminemia, hyperlipidemia, and edema. The pathophysiology 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 most common cause of nephrotic syndrome in children is minimal change disease (MCD), also known as lipid nephrosis. This condition is characterized by effacement of the foot processes of the podocytes on electron microscopy. The exact pathogenesis of MCD is not fully understood, but it is believed to involve immune-mediated damage to the glomerular basement membrane. The loss of albumin and other proteins leads to hypoalbuminemia, which in turn causes increased capillary permeability and edema.

**Why Each Wrong Option is Incorrect**
* **Option A:** Focal segmental glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome in adults, but it is less common in children. FSGS is characterized by scarring of the glomeruli, which is not typical of MCD.
* **Option B:** Membranous nephropathy is a common cause of nephrotic syndrome in adults, but it is relatively rare in children. This condition is characterized by thickening of the glomerular basement membrane.
* **Option C:** Membranoproliferative glomerulonephritis is a type of glomerulonephritis that can cause nephrotic syndrome, but it is not the most common cause in children. This condition is characterized by proliferation of the glomerular cells and thickening of the basement membrane.

**Clinical Pearl / High-Yield Fact**
MCD is often associated with a good response to corticosteroid therapy, and the majority of children with this condition will go into remission within a few months. However, some children may require prolonged treatment with corticosteroids or other immunosuppressive agents.

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