**Core Concept**
Henoch-Schönlein Purpura (HSP) is a form of vasculitis that affects small blood vessels, and it can involve various organs, including the skin, joints, gastrointestinal tract, and kidneys. The renal involvement in HSP is a significant concern due to the potential for long-term kidney damage. **Renal impairment** in HSP can range from mild hematuria to severe kidney failure.

**Why the Correct Answer is Right**
The correct answer is related to the percentage of patients with HSP who develop renal involvement. Approximately 50% of patients with HSP develop some form of **renal involvement**, which can include hematuria, proteinuria, or nephritic syndrome. The mechanism of renal involvement in HSP is thought to be related to the deposition of immune complexes in the glomeruli, leading to inflammation and damage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the frequency of renal involvement is higher than this.
**Option B:** This option is incorrect because the frequency of renal involvement is lower than this.
**Option D:** This option is incorrect because the frequency of renal involvement is not this high.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that renal involvement in HSP can be a significant predictor of long-term outcomes, and patients with severe renal involvement are at higher risk of developing **end-stage renal disease**.

**Correct Answer:** C. 50%