Correct Answer: D. Henoch Schoenlein purpura (HSP)
Henoch Schönlein Purpura (HSP) is the most common systemic vasculitis in children, presenting with a classic tetrad: palpable purpura (non-blanching, typically on lower extremities and buttocks), arthritis/arthralgia (knees and ankles), abdominal pain, and renal involvement. The discriminating feature here is the combination of non-blanching rash + knee joint swelling + haematuria + normal platelets. HSP is an IgA-mediated small-vessel vasculitis affecting skin, joints, GI tract, and kidneys. The normal platelet count rules out thrombocytopenic causes. Haematuria with proteinuria indicates IgA nephropathy (the renal manifestation of HSP), which occurs in 40–50% of HSP cases and is the most common cause of post-infectious glomerulonephritis in Indian children. The palpable purpura is pathognomonic—it does not blanch on pressure because it represents true vasculitis with RBC extravasation, not simple petechiae. HSP typically follows upper respiratory tract infection by 1–4 weeks, though the question does not specify this. Diagnosis is clinical; skin or kidney biopsy showing IgA deposition on immunofluorescence confirms it. Management in India follows IAP guidelines: supportive care, NSAIDs for arthritis, corticosteroids for severe manifestations (abdominal pain, renal involvement).
Why the other options are wrong
A. Immune thrombocytopenic purpura (ITP) — ITP presents with thrombocytopenia (low platelet count), but this child has normal platelet levels. ITP causes petechiae and purpura due to bleeding from low platelets, not vasculitis. There is no joint swelling or renal involvement in ITP. This is a classic NBE trap—both cause purpura, but platelet count is the discriminator. B. Systemic lupus erythematosus (SLE) — SLE can present with rash, arthritis, and renal disease (lupus nephritis), but it is rare in 9-year-old boys (SLE predominantly affects females, peak age 15–45 years). SLE rash is typically malar (butterfly), photosensitive, or discoid—not palpable purpura. HSP is far more common in this age group and gender. SLE would require positive ANA and anti-dsDNA antibodies. C. Polyarteritis Nodosa (PAN) — PAN is a medium-vessel vasculitis affecting coronary, renal, and mesenteric arteries, causing myocardial infarction, hypertension, and GI ischemia—not typical joint swelling or palpable purpura. PAN is rare in children and typically presents with systemic illness, fever, and organ-specific infarction. HSP (small-vessel vasculitis) is the correct vasculitis for this age and presentation.
High-Yield Facts
- HSP is the most common systemic vasculitis in children (peak age 4–6 years, rare after 10 years); presents with palpable purpura + arthritis + abdominal pain + renal involvement.
- Non-blanching palpable purpura on lower extremities and buttocks is pathognomonic for HSP; caused by IgA-mediated small-vessel vasculitis, not thrombocytopenia.
- Normal platelet count in HSP rules out ITP and thrombotic microangiopathies; haematuria + proteinuria indicate IgA nephropathy (40–50% of HSP cases).
- IgA deposition on skin or kidney biopsy (immunofluorescence) confirms HSP diagnosis; often follows URTI by 1–4 weeks.
- Indian DOC management: NSAIDs for arthritis, corticosteroids (prednisolone 1–2 mg/kg/day) for severe abdominal pain or progressive renal disease; most cases resolve spontaneously within 4–6 weeks.
Mnemonics
HSP Tetrad: PAAR Palpable purpura (non-blanching, lower limbs) | Arthritis (knees, ankles) | Abdominal pain | Renal involvement (haematuria, proteinuria). Use when you see any two features in a child—think HSP first. Why HSP ≠ ITP: Platelets Tell the Tale ITP = Low platelets (petechiae from bleeding). HSP = Normal platelets (purpura from vasculitis). If platelets are normal and there's a rash + joints + urine findings, HSP wins.
NBE Trap
NBE pairs "purpura + low platelets" (ITP) with "purpura + normal platelets" (HSP) to trap students who focus only on the rash without checking the platelet count. The normal platelet level is the key discriminator.
Clinical Pearl
In Indian pediatric practice, HSP is the leading cause of post-infectious glomerulonephritis and the most common vasculitis presenting to OPD. A child with palpable purpura on the buttocks and lower limbs + knee swelling + haematuria should trigger immediate HSP diagnosis and renal follow-up, as progressive IgA nephropathy can lead to chronic kidney disease if not monitored.
_Reference: OP Ghai Textbook of Pediatrics, Ch. Vasculitis in Children; IAP Clinical Practice Guidelines on HSP_