**Core Concept**
Henoch-Schönlein purpura (HSP) is a form of vasculitis that affects small blood vessels, characterized by the deposition of immune complexes in the vessel walls. This leads to inflammation and damage to the vessels, resulting in the classic symptoms of HSP, including purpura, arthritis, gastrointestinal symptoms, and renal involvement.

**Why the Correct Answer is Right**
The correct answer is IgA. In HSP, IgA immune complexes deposit in the walls of small blood vessels, including capillaries, venules, and arterioles. This deposition of IgA immune complexes triggers an inflammatory response, leading to the characteristic symptoms of HSP. The IgA immune complexes activate the complement system, which further exacerbates the inflammatory response and contributes to the damage of the vessel walls.

**Why Each Wrong Option is Incorrect**
* **Option A:** IgG - While IgG is the most common antibody isotype in the human body, it is not the primary antibody involved in the pathogenesis of HSP.
* **Option B:** IgM - IgM is also not typically associated with the pathogenesis of HSP.
* **Option D:** C3 - C3 is a component of the complement system, but it is not the primary antibody involved in the deposition in HSP.

**Clinical Pearl / High-Yield Fact**
HSP is a classic example of a condition that affects small blood vessels and is associated with IgA deposition. It is often seen in children and young adults, but can also occur in older adults. A high index of suspicion and prompt recognition of the condition are essential for effective management and prevention of complications.

**Correct Answer: C. IgA. IgA antibodies.**