A 7-year-old boy presents with sudden onset of a palpable, non-thrombocytopenic purpuric rash over his buttocks and lower extremities, crampy abdominal pain, and swelling in his bilateral ankles. Complete blood count reveals a normal platelet count. Urinalysis shows microscopic hematuria and proteinuria. What is the diagnosis?

Answer and explanation

Correct answer: D. Henoch-Schönlein purpura (IgA vasculitis)

  1. Step 1: Analyze clinical quartet: Palpable purpura (without thrombocytopenia), colicky abdominal pain, arthritis/arthralgia, and renal involvement (hematuria).
  2. Step 2: Recognize pathophysiological hallmark: IgA immune complex deposition in small vessel walls characterizes Henoch-Schönlein purpura (IgA vasculitis).
  3. Step 3: Distinguish from ITP: Platelet count is normal in Henoch-Schönlein purpura, ruling out immune thrombocytopenia.
Answer
Henoch-Schönlein purpura (IgA vasculitis).
Why not the others: 'Immune thrombocytopenic purpura' features profound thrombocytopenia (platelets often ); 'Meningococcemia' presents with toxic septic shock and purpura fulminans; 'HUS' features the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure.

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