## Core Concept
The patient's presentation of generalized edema, marked albuminuria, hypoalbuminemia, and hyperlipidemia is suggestive of **nephrotic syndrome**. Nephrotic syndrome is characterized by heavy proteinuria (>3.5 g/1.73 m²/24 hr), hypoalbuminemia, hyperlipidemia, and edema. The condition can result from various glomerular diseases.

## Why the Correct Answer is Right
The correct answer, **podocyte effacement**, is a hallmark ultrastructural finding in **minimal change disease (MCD)**, which is the most common cause of nephrotic syndrome in children. On light microscopy, the kidney appears normal, which is consistent with MCD. Electron microscopy (EM) in MCD shows **fusion and effacement of podocyte foot processes**, leading to loss of the normal filtration barrier and massive proteinuria.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not specify a known ultrastructural finding associated with nephrotic syndrome or MCD.
* **Option B:** This option is incorrect as it does not describe a characteristic EM finding in the context of nephrotic syndrome or glomerular diseases leading to this condition.
* **Option C:** This option is incorrect because, although mesangial proliferation can be seen in some glomerulonephritides, it does not specifically relate to the EM findings expected in MCD or typical nephrotic syndrome presentations.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **minimal change disease** is highly responsive to **corticosteroids**, with most children achieving remission within 8-12 weeks of treatment. The disease's excellent response to steroids is a critical distinguishing feature and a significant prognostic indicator.

## Correct Answer Line
**Correct Answer: D. podocyte effacement**.