**Core Concept**
The underlying condition being tested is a renal disorder characterized by significant proteinuria, hypoalbuminemia, and edema, likely **nephrotic syndrome**. This condition involves damage to the glomeruli, leading to excessive loss of protein in the urine.

**Why the Correct Answer is Right**
Given the child's response to glucocorticoid therapy and the presence of nephrotic syndrome, the most likely diagnosis is **minimal change disease (MCD)**. MCD is characterized by the loss of foot processes of the podocytes as seen under electron microscopy, but the glomeruli appear normal under light microscopy. This condition is the most common cause of nephrotic syndrome in children.

**Why Each Wrong Option is Incorrect**
**Option A:** This might represent a different glomerular disease, not consistent with the clinical presentation and response to treatment described.
**Option B:** This could be indicative of another renal condition, such as membranous nephropathy, which does not typically respond as well to glucocorticoids as MCD.
**Option C:** This option might suggest a condition like focal segmental glomerulosclerosis (FSGS), which can present similarly but often has a different response to steroid therapy.
**Option D:** This is not characteristic of the disease process described, which is responsive to glucocorticoids.

**Clinical Pearl / High-Yield Fact**
The key point to remember is that **minimal change disease** is the most common cause of nephrotic syndrome in children and typically shows a good response to glucocorticoid therapy. The diagnosis is often made based on the clinical presentation and the absence of other findings, with electron microscopy showing effacement of podocyte foot processes.

**Correct Answer:** D. Effacement of foot processes