A 7-year-old boy presents with a 2-week history of symmetric palpable purpura on the lower extremities and buttocks following a recent upper respiratory infection. He also complains of colicky abdominal pain and ankle arthralgia. On examination, the lesions marked **A** in the diagram are non-blanching and do not blanch with pressure. His platelet count is 250,000/µL (normal). Based on the clinical presentation and the characteristic appearance of the lesions marked **A**, what is the most likely diagnosis?

Explanation

## Why Henoch-Schönlein Purpura (IgA vasculitis) is right The clinical tetrad of palpable purpura (mandatory feature), abdominal pain, arthralgia, and renal involvement (evidenced by urinalysis screening) is pathognomonic for Henoch-Schönlein Purpura. The lesions marked **A** — palpable purpuric papules — are the hallmark of small-vessel leukocytoclastic vasculitis with IgA1 immune complex deposition in vessel walls. Critically, the normal platelet count (250,000/µL) rules out thrombocytopenic causes and confirms vasculitis as the mechanism. According to EULAR/PRINTO/PReS 2010 criteria, palpable purpura is mandatory; the presence of ≥1 additional feature (here: abdominal pain, arthralgia) confirms diagnosis. HSP is the most common vasculitis of childhood, with peak incidence at 4–7 years and winter predominance (Nelson Textbook of Pediatrics 21st ed). ## Why each distractor is wrong - **Immune Thrombocytopenic Purpura (ITP)**: ITP presents with petechiae and ecchymoses due to thrombocytopenia (platelet count <150,000/µL). This patient has a normal platelet count and palpable purpuric papules (not petechiae), which are characteristic of vasculitis, not thrombocytopenia. ITP does not cause abdominal pain, arthralgia, or renal involvement. - **Acute Leukemia with thrombocytopenia**: Acute leukemia may present with purpura and abdominal pain, but the platelet count is typically severely depressed (<50,000/µL). This patient's normal platelet count and the symmetric, dependent distribution of palpable purpura on extensor surfaces are inconsistent with leukemia. - **Meningococcal septicemia**: While meningococcal septicemia can present with purpura fulminans and systemic symptoms, it is a medical emergency with signs of septic shock, meningitis, and rapidly progressive non-blanching petechiae. The symmetric, dependent distribution on lower extremities and buttocks, combined with the subacute course and normal platelets, is more consistent with HSP. **High-Yield:** Palpable purpura + normal platelets + abdominal pain + arthralgia in a child = Henoch-Schönlein Purpura until proven otherwise; always check urinalysis for hematuria/proteinuria to assess renal involvement. [cite: EULAR/PRINTO/PReS Criteria 2010; Nelson Textbook of Pediatrics 21st ed]