**Core Concept**
Minimal change glomerulonephritis (MC GN) is a common cause of nephrotic syndrome in children. It is characterized by the loss of large amounts of protein in the urine due to damage to the glomerular filtration barrier. The pathophysiology of MC GN involves changes in the podocytes, leading to the loss of their foot processes and disruption of the glomerular basement membrane.

**Why the Correct Answer is Right**
The correct answer, **Option B: IgG deposition in mesangium**, is incorrect because MC GN is typically characterized by the absence of immune deposits, including IgG, in the glomeruli. In contrast, other forms of glomerulonephritis, such as membranous nephropathy, are associated with IgG deposition. The lack of immune deposits in MC GN is a key feature that distinguishes it from other forms of glomerulonephritis.

**Why Each Wrong Option is Incorrect**
**Option A:** Selective proteinuria is actually a feature of MC GN, as the disease is characterized by the loss of albumin and other small proteins in the urine. This is due to the damage to the glomerular filtration barrier, which allows large molecules like albumin to pass through.
**Option C:** MC GN is indeed most common in children aged 2–9 years, making this statement true.
**Option D:** MC GN does respond to steroids, which are the first-line treatment for this condition. Steroids help to reduce proteinuria and improve kidney function in most children with MC GN.

**Clinical Pearl / High-Yield Fact**
The hallmark of MC GN is the presence of "minimal change" on light microscopy, with normal-appearing glomeruli. However, electron microscopy reveals podocyte foot process effacement, which is a key feature of this condition.

**✓ Correct Answer: B. IgG deposition in mesangium**