**Core Concept**
The child's presentation of non-blanching macules and papules on the lower extremities, accompanied by mild abdominal pain and IgA deposition on skin biopsy, suggests a diagnosis of Henoch-Schönlein purpura (HSP). HSP is a form of vasculitis that affects small blood vessels and is characterized by the deposition of IgA immune complexes.

**Why the Correct Answer is Right**
Henoch-Schönlein purpura is a small vessel vasculitis that typically affects children and young adults. The deposition of IgA immune complexes in the skin and other organs leads to inflammation and damage to the blood vessels, resulting in the characteristic purpuric rash and abdominal pain. The IgA deposition on skin biopsy is a diagnostic hallmark of HSP. The involvement of the skin, gut, and kidneys in HSP is due to the widespread deposition of IgA immune complexes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not mention the characteristic IgA deposition seen in HSP.
**Option B:** This option is incorrect because it does not specify the involvement of small blood vessels, which is a key feature of HSP.
**Option C:** This option is incorrect because it does not mention the typical age group and presentation of HSP.

**Clinical Pearl / High-Yield Fact**
HSP often presents with a characteristic "tetrad" of symptoms: non-thrombocytopenic purpura, abdominal pain, renal involvement, and arthritis. The presence of IgA deposition on skin biopsy is a diagnostic clue to HSP.

**Correct Answer: D. Henoch-Schönlein purpura**