**Core Concept**
En Henoch-Scholein purpura (HSP) is a form of vasculitis that primarily affects the small blood vessels. Renal involvement in HSP is a significant concern and can lead to severe complications. The timing of renal involvement is crucial in determining the prognosis and management of the disease.

**Why the Correct Answer is Right**
Renal involvement in HSP is typically seen after the onset of skin manifestations, usually within 1-3 weeks. This is because the immune complex deposition and inflammation in the kidneys occur as a result of the systemic vasculitis. The kidneys are affected when the immune complexes deposit in the glomeruli, leading to glomerulonephritis. If there is no renal involvement till 7 days after the onset of skin manifestations, it is less likely to occur.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because renal involvement can occur even before the skin manifestations in some cases.
* **Option B:** This is incorrect because renal involvement can occur even after 14 days of skin manifestations.
* **Option C:** This is incorrect because renal involvement can occur even after 7 days of skin manifestations in some cases.

**Clinical Pearl / High-Yield Fact**
It is essential to closely monitor patients with HSP for signs of renal involvement, especially within 1-3 weeks of skin manifestations.

**Correct Answer:** D. 7 days