**Core Concept**
Hemolytic uremic syndrome (HUS) is a complication of Henoch-Schönlein purpura (HSP), an IgA-mediated vasculitis. Renal involvement in HSP is characterized by the deposition of IgA immune complexes in the glomeruli, leading to inflammation and potential renal damage.

**Why the Correct Answer is Right**
The frequency of renal involvement in HSP is estimated to be between 40-60%. This range is consistent with the pathophysiology of HUS, where the deposition of IgA immune complexes in the glomeruli leads to inflammation and potential renal damage. The involvement of the kidneys in HSP is a significant clinical concern, as it can lead to acute kidney injury and long-term renal damage if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** 20-40% is an underestimation of the true frequency of renal involvement in HSP. While some patients may not develop renal symptoms, the majority of those with HSP will experience some degree of renal involvement.

**Option B:** >80% is an overestimation of the true frequency of renal involvement in HSP. While renal involvement is a common complication of HSP, the majority of patients will not develop severe renal symptoms.

**Option D:** 10% is a gross underestimation of the true frequency of renal involvement in HSP. Renal involvement is a significant clinical concern in HSP, and the majority of patients will experience some degree of renal symptoms.

**Clinical Pearl / High-Yield Fact**
It is essential to note that renal involvement in HSP can occur at any age, but it is more common in children. Early recognition and treatment of renal involvement in HSP are crucial to prevent long-term renal damage.

**✓ Correct Answer: C. 40-60%.**