**Core Concept**
The child's presentation of non-blanching macules and papules, mild abdominal pain, and IgA deposition on skin biopsy is suggestive of a vasculitic process involving small vessels. This is a classic presentation of Henoch-Schönlein purpura (HSP), an IgA-mediated small vessel vasculitis.

**Why the Correct Answer is Right**
Henoch-Schönlein purpura is characterized by deposition of IgA immune complexes in the walls of small blood vessels, leading to inflammation and resultant skin, gastrointestinal, and renal manifestations. The presence of non-blanching macules and papules, along with abdominal pain, is consistent with this diagnosis. The skin biopsy showing IgA deposition is diagnostic of HSP.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect as it does not match the clinical presentation and biopsy findings of the child. HSP is an IgA-mediated vasculitis, whereas this option suggests a different underlying mechanism.

**Option B:** This is incorrect as it does not align with the biopsy findings of IgA deposition. HSP is a distinct clinical entity characterized by IgA-mediated small vessel vasculitis.

**Option C:** This is incorrect as it does not match the clinical presentation of non-blanching macules and papules, abdominal pain, and IgA deposition. This option suggests a different underlying condition.

**Clinical Pearl / High-Yield Fact**
HSP is a classic example of an IgA-mediated small vessel vasculitis, often presenting with non-blanching macules and papules, abdominal pain, and renal involvement. It is essential to recognize this condition, as prompt treatment can significantly improve outcomes.

**Correct Answer:** C.