**Core Concept**
The patient's presentation of palpable purpura, arthralgias, abdominal pain with diarrhea and passage of blood per rectum, along with proteinuria, is suggestive of a systemic vasculitis involving small vessels, specifically Henoch-Schönlein purpura (HSP). HSP is a form of vasculitis that is characterized by the deposition of IgA immune complexes in the walls of small blood vessels.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with the classic pentad of HSP: palpable purpura, abdominal pain, arthralgias, gastrointestinal bleeding, and renal involvement (proteinuria). The deposition of IgA immune complexes in the walls of small blood vessels leads to inflammation and damage, resulting in the characteristic symptoms of HSP. The presence of proteinuria indicates renal involvement, which is a common complication of HSP.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be a distractor for other forms of vasculitis, such as Kawasaki disease or polyarteritis nodosa, but the patient's presentation is more specific to HSP.
**Option B:** This option might be a distractor for other conditions that cause abdominal pain and diarrhea, such as inflammatory bowel disease, but the presence of palpable purpura and proteinuria points more towards HSP.
**Option C:** This option might be a distractor for other conditions that cause renal involvement, such as lupus nephritis, but the patient's presentation is more specific to HSP.
**Option D:** This option might be a distractor for other forms of vasculitis, such as giant cell arteritis, but the patient's presentation is more specific to HSP.

**Clinical Pearl / High-Yield Fact**
HSP is a common cause of renal involvement in children, and the presence of proteinuria is a significant predictor of long-term renal outcome. Patients with HSP should be closely monitored for signs of renal involvement and promptly treated if renal damage is suspected.

**Correct Answer:** C. Henoch-Schönlein purpura (HSP)