**Core Concept**
Henoch-Schönlein purpura (HSP) is a form of vasculitis characterized by the deposition of IgA immune complexes in small blood vessels, leading to inflammation and bleeding. This condition typically affects children and young adults, presenting with a tetrad of symptoms: non-thrombocytopenic purpura, arthritis, abdominal pain, and renal involvement.

**Why the Correct Answer is Right**
The correct answer is associated with the characteristic skin lesions in HSP. The purpuric rash in HSP is a result of the deposition of IgA immune complexes in the walls of small blood vessels, leading to inflammation and bleeding. This results in palpable purpura, which is a hallmark feature of the condition. The correct answer is also related to the specific location of the purpura, which tends to occur on the lower extremities, buttocks, and sometimes the face.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while HSP can involve the upper respiratory tract, it is not a characteristic feature of the condition. HSP is more commonly associated with gastrointestinal symptoms such as abdominal pain.
**Option B:** This option is incorrect because while joint pain is a common feature of HSP, it is not typically associated with large joint involvement. Small joint involvement is more characteristic of the condition.
**Option C:** This option is incorrect because while renal involvement can occur in HSP, it is not a universal feature of the condition. Renal involvement occurs in approximately 50% of cases.

**Clinical Pearl / High-Yield Fact**
It's essential to note that HSP can be a precursor to IgA nephropathy, a common cause of chronic kidney disease. Therefore, patients with a history of HSP should be monitored for signs of renal involvement.

**Correct Answer: D. Non-thrombocytopenic purpura.**