A 14-year-old boy presents to the hospital with severe leg swelling that started 2 weeks ago. He also notes feeling tired and having little energy to play sports with his friends. His past medical history is negative and he is not taking any medications.On examination, his blood pressure is 163/96 mm Hg, and there is pedal edema up to his knees, as well as periorbital edema. His remaining clinical exam is normal. A urinalysis is positive for 3+ proteinuria and on 24-hour urine collection the total protein excretion is 5.4 grams/day. He undergoes a renal biopsy and there are no changes seen on light microscopy, but electron microscopy shows foot process fusion and no deposits on the membranes. Which of the following is the most likely diagnosis?

**Core Concept**
The patient presents with nephrotic syndrome, characterized by severe proteinuria (>3.5 grams/day), hypoalbuminemia, hyperlipidemia, and edema. The absence of changes on light microscopy, but presence of foot process fusion on electron microscopy, suggests a specific underlying cause of the disease.

**Why the Correct Answer is Right**
The patient's presentation and biopsy findings are consistent with minimal change disease (MCD), a common cause of nephrotic syndrome in children. The foot process fusion on electron microscopy is a hallmark feature of MCD. The disease is thought to result from an immune-mediated injury to the podocytes, leading to the loss of foot processes and subsequent severe proteinuria. The lack of changes on light microscopy is due to the fact that the injury to the podocytes is subtle and not visible under light microscopy.

**Why Each Wrong Option is Incorrect**
**Option A:** Focal segmental glomerulosclerosis (FSGS) is another cause of nephrotic syndrome, but it typically shows changes on light microscopy, such as segmental sclerosis and collapse of glomeruli.
**Option B:** Membranous nephropathy is a cause of nephrotic syndrome, but it is characterized by the presence of immune deposits on the glomerular basement membrane, which are not seen on electron microscopy in this patient.
**Option C:** Membranoproliferative glomerulonephritis is a cause of nephrotic syndrome, but it typically shows changes on light microscopy, such as thickened glomerular basement membrane and mesangial proliferation.

**Clinical Pearl / High-Yield Fact**
Minimal change disease is the most common cause of nephrotic syndrome in children and often responds well to corticosteroid therapy. The disease is thought to result from an immune-mediated injury to the podocytes, leading to the loss of foot processes and subsequent severe proteinuria.

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

✓ Correct Answer: B. minimal change disease