**Core Concept**
The clinical presentation suggests a systemic condition affecting the skin, joints, and kidneys, likely indicating a **vasculitis** or **nephritic syndrome**. The combination of non-blanching rashes (purpura), joint swelling, and renal involvement (hematuria and proteinuria) points towards an immune-mediated process.

**Why the Correct Answer is Right**
Given the symptoms, the most likely diagnosis is **Henoch-Schönlein Purpura (HSP)**, a form of **vasculitis** that affects small blood vessels. HSP is characterized by deposition of IgA immune complexes, leading to inflammation in various tissues, including the skin, joints, and kidneys. The renal biopsy in HSP would typically show **mesangial IgA deposition**, which is a hallmark of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not specifically relate to the clinical presentation of HSP.
**Option B:** Similarly, this option does not align with the expected findings in HSP.
**Option D:** This choice is also incorrect as it does not match the characteristic renal biopsy findings in HSP.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Henoch-Schönlein Purpura** is one of the most common forms of vasculitis in children, and its diagnosis is primarily clinical, supported by the presence of **IgA deposits in the renal biopsy**.

**Correct Answer:** D. Mesangial IgA deposition