**Core Concept**
The question is testing the student's understanding of the pathophysiological features of IgA nephropathy, a common cause of nephritic syndrome in children. IgA nephropathy is characterized by the deposition of IgA antibodies in the glomerular mesangium, leading to inflammation and renal damage.

**Why the Correct Answer is Right**
The correct answer is C. Mesangial deposits of IgA. This is because IgA nephropathy is a condition where IgA antibodies are deposited in the mesangial region of the glomerulus, leading to activation of the complement system and recruitment of inflammatory cells. This results in the characteristic clinical features of nephritic syndrome, including non-blanching rashes (purpura), hematuria, and proteinuria. The presence of IgA deposits in the mesangium is a hallmark of this condition and can be identified on immunofluorescence microscopy.

**Why Each Wrong Option is Incorrect**
**Option A:** Tubular necrosis is more commonly associated with acute tubular necrosis (ATN), which is a different condition characterized by damage to the renal tubules. While ATN can also present with hematuria and proteinuria, it is not typically associated with mesangial IgA deposits.

**Option B:** Visceral podocyte fusion is a feature of minimal change disease, a condition that typically presents with massive proteinuria and normal renal histology on light microscopy. While minimal change disease can also present with hematuria, the presence of mesangial IgA deposits is not characteristic of this condition.

**Option D:** Basement membrane thickening is a feature of membranous nephropathy, a condition that typically presents with massive proteinuria and normal renal histology on light microscopy. While membranous nephropathy can also present with hematuria, the presence of mesangial IgA deposits is not characteristic of this condition.

**Clinical Pearl / High-Yield Fact**
IgA nephropathy is a common cause of nephritic syndrome in children, and the presence of IgA deposits in the mesangium is a key diagnostic feature. The condition is often associated with upper respiratory tract infections, and the deposition of IgA antibodies in the glomerulus is thought to be triggered by an immune response to these infections.

**✓ Correct Answer: C. Mesangial deposits of IgA**