Okay, let's tackle this question. The patient is a 7-year-old with non-blanching rashes on the shins, knee joint swelling, hematuria, and some proteinuria. The key here is to think about the differential diagnoses. Non-blanching rashes, especially on the lower extremities, make me think of Henoch-Schönlein purpura (HSP). HSP is a vasculitis that commonly affects children and can present with palpable purpura, arthritis, abdominal pain, and renal involvement.

The renal findings here are hematuria and proteinuria, which are classic for HSP nephritis. Now, the question is about the renal biopsy findings. I remember that HSP nephritis is a IgA-mediated glomerulonephritis. So, in the biopsy, the main finding would be IgA deposition in the mesangium of the glomeruli. This is similar to other IgA nephropathies, but the clinical presentation of HSP with the rash and joints differentiates it.

Looking at other options: If the answer was something like membranoproliferative glomerulonephritis (MPGN), that's usually IgM and C3 deposits. Or if it's post-streptococcal glomerulonephritis, that's IgG and C3. Lupus nephritis would have immune complex deposits but with a different clinical context. So the IgA deposits in the mesangium are specific for HSP nephritis here.

The clinical pearl is to remember that IgA nephropathy and HSP nephritis are the same pathologically but different clinically. HSP presents with the classic tetrad: purpura, arthritis, abdominal pain, and renal disease. The biopsy shows IgA in the mesangium, which is key for diagnosis.

**Core Concept**
This question tests the pathophysiology of Henoch-Schönlein purpura (HSP) nephritis, a small-vessel vasculitis caused by IgA immune complex deposition. The clinical triad of palpable purpura, arthritis, and renal involvement (hematuria/proteinuria) is diagnostic of HSP, with renal biopsy revealing characteristic IgA mesangial deposition.

**Why the Correct Answer is Right**
In HSP, IgA1 autoantibodies form immune complexes that deposit in the mesangium of glomeruli, triggering inflammation. Renal biopsy shows **IgA-dominant mesangial deposits** on immunofluorescence, with light microscopy revealing mesangial proliferation and focal segmental endocapillary proliferation. This distinguishes it from other glomerulonephritides like post-streptococcal (IgG/C3) or lupus (mixed immune complexes).

**Why Each Wrong Option is Incorrect**
**Option A:** Membranoproliferative glomerulonephritis (MPGN) shows subendothelial and mesangial immune complex deposits (IgG, IgM, C3), not IgA.
**Option B:** Minimal change disease is a T-cell–mediated injury without immune deposits; it lacks hematuria and presents with nephrotic syndrome.
**Option C:** Focal segmental glomerulosclerosis (FSGS) shows sclerotic lesions on light

✓ Correct Answer: C. Mesangial deposits of IgA