**Core Concept**
Palpable purpura refers to the presence of visible, bleeding lesions on the skin, typically resulting from damage to small blood vessels. This condition is often associated with systemic vasculitis, an inflammatory process affecting blood vessels of various sizes.

**Why the Correct Answer is Right**
Palpable purpura can be caused by several forms of vasculitis, including Henoch-Schönlein purpura (HSP), Polyarteritis nodosa (PAN), and Microscopic polyangitis (MPA). HSP is characterized by IgA deposits in the affected vessels, leading to inflammation and damage. PAN involves the medium-sized vessels and is often associated with granulomatosis. MPA affects the small vessels and is often seen in patients with anti-neutrophil cytoplasmic antibodies (ANCA). The presence of palpable purpura in these conditions is a result of the inflammatory process damaging the vessel walls, leading to leakage of blood into the surrounding tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** While Henoch-Schönlein purpura (HSP) is a common cause of palpable purpura, it is not the only possible cause. Other forms of vasculitis, such as PAN and MPA, can also present with this symptom.
**Option B:** Polyarteritis nodosa (PAN) can cause palpable purpura, but it is not the only possible cause. HSP and MPA can also present with this symptom.
**Option C:** Microscopic polyangitis (MPA) can cause palpable purpura, but it is not the only possible cause. HSP and PAN can also present with this symptom.

**Clinical Pearl / High-Yield Fact**
In patients presenting with palpable purpura, it is essential to consider the possibility of systemic vasculitis. A thorough evaluation, including laboratory tests and imaging studies, is necessary to determine the underlying cause and guide appropriate management.

**✓ Correct Answer: D. All of the above**