A 6-year-old boy presents with a 2-week history of upper respiratory tract infection followed by a rash. Examination reveals the finding marked **A** in the diagram — palpable purpura over the extensor surfaces of the lower limbs and buttocks. His platelet count is 250,000/μL. Which of the following is the most likely diagnosis?

Explanation

## Why Henoch-Schönlein Purpura (IgA Vasculitis) is right The clinical presentation of palpable purpura over the extensor lower limbs and buttocks (marked **A**) in a child aged 6 years, preceded by URI, with normal platelet count, is pathognomonic for Henoch-Schönlein Purpura (HSP/IgA vasculitis). HSP is the most common small vessel vasculitis of childhood (peak 3–10 years, M:F 2:1) and is characterized by IgA1 immune complex deposition in small vessel walls causing leukocytoclastic vasculitis. The palpable purpura is non-thrombocytopenic (normal platelets rule out ITP) and shows gravity-dependent distribution over lower limbs and buttocks, never involving the face. The preceding URI (often Group A Strep, mycoplasma, or parvovirus B19) within 1–3 weeks is a classic trigger. According to Nelson 21e, the EULAR/PRINTO/PRES diagnostic criteria require palpable purpura plus at least one additional feature (arthritis, abdominal pain, or renal disease) — the purpura alone with this distribution is sufficient for diagnosis. ## Why each distractor is wrong - **Immune Thrombocytopenic Purpura (ITP)**: ITP presents with thrombocytopenia (low platelets), whereas this patient has normal platelet count (250,000/μL). ITP purpura is typically non-palpable and petechial, not the palpable purpura seen here. The clinical context of URI prodrome and lower limb distribution is not typical for ITP. - **Acute Post-Streptococcal Glomerulonephritis**: While this can follow streptococcal infection, it does not present with palpable purpura as the primary finding. APSGN manifests with hematuria, hypertension, and edema, not a characteristic rash. The purpura in **A** is the dominant feature here. - **Meningococcal Septicemia**: Meningococcal purpura is typically non-palpable, petechial, and rapidly progressive with systemic toxicity, fever, and meningeal signs. The gradual onset after URI and the gravity-dependent distribution over lower limbs (not generalized) are inconsistent with meningococcemia. **High-Yield:** HSP = palpable purpura (lower limbs + buttocks, never face) + normal platelets + URI prodrome (1–3 weeks prior) = most common small vessel vasculitis in children. [cite: Nelson Textbook of Pediatrics, 21st edition, Chapter 192]