**Core Concept**
Henoch-Schönlein Purpura (HSP) is a form of vasculitis that affects small blood vessels, and renal involvement is a significant complication. The underlying principle being tested is the frequency of renal manifestations in HSP.

**Why the Correct Answer is Right**
The correct answer is based on the pathophysiology of HSP, where immune complex deposition leads to inflammation of the small blood vessels, including those in the kidneys. This can result in renal impairment, ranging from mild hematuria to severe kidney disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it underestimates the frequency of renal involvement.
**Option B:** This option overestimates the frequency, as not all HSP patients develop renal symptoms.
**Option D:** This option is also incorrect, as it does not accurately reflect the known frequency of renal involvement in HSP.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that renal involvement in HSP can be unpredictable and may progress to severe kidney disease, emphasizing the need for close monitoring.

**Correct Answer:** C. 50%