A 7-year-old boy presents two weeks after a sore throat with a one-week history of crops of non-blanching purpura on the buttocks and extensor surfaces of the lower limbs. He also complains of colicky periumbilical abdominal pain and swelling of both ankles and knees. Urinalysis shows microscopic hematuria. Platelet count is normal (245,000/μL). The clinical presentation shown in structure **B** is most consistent with which of the following pathophysiological mechanisms?
A. Type III hypersensitivity reaction to drug haptens with self-limited cutaneous vasculitis and normal platelet count
B. Deposition of galactose-deficient IgA1-containing immune complexes in small-vessel walls of skin, gut, joints, and kidney glomeruli
C. Direct endothelial injury from meningococcal lipopolysaccharide with rapid thrombosis and vessel occlusion
D. Circulating immune complexes of IgG and IgM with complement activation causing small-vessel vasculitis
Explanation
Why "Deposition of galactose-deficient IgA1-containing immune complexes..." is right
The clinical presentation marked B — palpable purpura on dependent areas (buttocks and lower limb extensor surfaces), arthritis, abdominal pain, and renal involvement (hematuria) — is the classic tetrad of Henoch-Schönlein purpura (HSP), now classified as IgA vasculitis. The pathophysiology is characterized by deposition of IgA1-containing immune complexes, specifically galactose-deficient IgA1, in small-vessel walls of skin, gut, joints, and kidney glomeruli. This is identical to the immunopathology of IgA nephropathy. Skin biopsy would show leukocytoclastic vasculitis with IgA deposits on direct immunofluorescence, confirming the diagnosis. HSP is the most common systemic vasculitis in children (peak age 3–10 years) and typically follows upper respiratory tract infections.
Why each distractor is wrong
Circulating immune complexes of IgG and IgM with complement activation: This describes other forms of small-vessel vasculitis (e.g., serum sickness-like reactions or post-infectious vasculitis) but not HSP. HSP is specifically IgA-mediated, not IgG/IgM-mediated. The immunofluorescence pattern would be different.
Direct endothelial injury from meningococcal lipopolysaccharide: This describes meningococcemia (structure C), which presents with rapidly progressive non-blanching purpura and shock. Meningococcemia is a medical emergency with systemic toxicity, whereas HSP is typically self-limited and the child is well-appearing.
Type III hypersensitivity reaction to drug haptens: This describes hypersensitivity vasculitis (structure A), which presents with self-limited cutaneous purpura only, without systemic involvement (no arthritis, abdominal pain, or renal disease). Drug-induced vasculitis does not produce the multisystem tetrad seen in HSP.
High-YieldNEET PG
HSP = IgA vasculitis with galactose-deficient IgA1 immune complex deposition; tetrad = palpable purpura (lower limbs/buttocks) + arthritis + abdominal pain + renal involvement; most common systemic vasculitis in children.
EULAR/PRES 2010 Criteria; Nelson Textbook of Pediatrics 21e
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