A 4-year-old girl presents with swelling of the legs and ankles. Physical examination reveals pitting edema of the lower extremities. Urinalysis show 2+ proteinuria. The urinary sediment contains no inflammatory cells or red blood cells. Serum levels of BUN and creatinine are normal. The patient recovers completely after a course of corticosteroids. For the patient, electron microscopy of a renal biopsy specimen prior to treatment would most likely demonstrate which of the following abnormalities?

**Core Concept**
The patient's presentation of pitting edema, proteinuria without hematuria, and response to corticosteroids is suggestive of nephrotic syndrome, a clinical entity characterized by massive proteinuria, hypoalbuminemia, and edema.

**Why the Correct Answer is Right**
The patient's symptoms and response to corticosteroids are consistent with minimal change disease (MCD), a common cause of nephrotic syndrome in children. Electron microscopy of a renal biopsy specimen in MCD typically reveals effacement of the podocyte foot processes, which is indicative of the disease's pathophysiology. This effacement disrupts the normal glomerular filtration barrier, leading to massive proteinuria.

**Why Each Wrong Option is Incorrect**
**Option A:** Focal segmental glomerulosclerosis (FSGS) is another cause of nephrotic syndrome, but it typically presents with more severe renal dysfunction and is less responsive to corticosteroids.

**Option B:** Membranous nephropathy is a cause of nephrotic syndrome, but it is more common in adults and often presents with more severe proteinuria and renal dysfunction.

**Option C:** Membranoproliferative glomerulonephritis is a cause of nephrotic syndrome, but it typically presents with hematuria and renal dysfunction, and is less responsive to corticosteroids.

**Clinical Pearl / High-Yield Fact**
Minimal change disease is the most common cause of nephrotic syndrome in children, and responds well to corticosteroids. Electron microscopy is essential for diagnosing MCD, as it reveals the characteristic effacement of podocyte foot processes.

**Correct Answer:** C. Effacement of the podocyte foot processes.

✓ Correct Answer: D. Fusion of podocyte foot processes