**Core Concept**
Henoch-Schönlein purpura (HSP) is a form of vasculitis characterized by the deposition of IgA immune complexes in small blood vessels, leading to inflammation and damage. This condition typically affects children and young adults, presenting with a classic triad of palpable purpura, abdominal pain, and arthralgias.

**Why the Correct Answer is Right**
The laboratory findings in this case are consistent with HSP, including mild leucocytosis, eosinophilia, normal platelet count, and elevated IgA levels. The presence of proteinuria and microscopic haematuria suggests kidney involvement, which is a common complication of HSP. The skin biopsy specimen would likely show deposition of IgA immune complexes in the dermal blood vessels, confirming the diagnosis. The absence of frank arthritis, normal PT and aPTT, and normal serum complement components also supports the diagnosis of HSP.

**Why Each Wrong Option is Incorrect**
**Option A:** A clotting disorder would typically present with abnormalities in PT and aPTT, which are normal in this case.
**Option B:** Septic emboli would likely present with fever, leukocytosis, and abnormalities in blood cultures, which are not mentioned in the case.
**Option D:** Urticarial vasculitis is a form of vasculitis that presents with urticarial lesions, which are not described in the case.

**Clinical Pearl / High-Yield Fact**
HSP is often associated with gastrointestinal symptoms, including abdominal pain, nausea, vomiting, and diarrhea, which can be severe enough to mimic an acute abdomen. A high index of suspicion is necessary to diagnose HSP in children presenting with these symptoms.

**✓ Correct Answer: C. HSP**