What is the most likely diagnosis for a 16-year-old boy presenting with palpable purpura, abdominal pain, joint aches, and moderate proteinuria in the urine?

Diagnosis: Henoch-Schönlein Purpura

The most likely diagnosis is Henoch-Schönlein purpura (HSP), based on the presence of palpable purpura extending to the buttocks, abdominal pain with bloody stools, bilateral joint involvement (knee aches), and moderate proteinuria on urinalysis. 1, 2, 3

Diagnostic Reasoning

This 16-year-old boy meets the established diagnostic criteria for HSP, which requires palpable purpura plus at least one of the following: renal involvement (hematuria and/or proteinuria), arthralgia/arthritis, or abdominal pain. 1, 2, 3 He demonstrates all three additional criteria beyond the purpura:

• Renal involvement: Moderate proteinuria on urinalysis 1, 2
• Joint involvement: Bilateral knee aches (arthralgia) 1, 2
• Gastrointestinal involvement: Abdominal pain with bloody loose bowel motions 1, 2, 3

The classic triad of hematuria, purpuric lesions, and ankle/joint pain is specifically diagnostic of HSP. 2 The distribution of the rash extending up to the buttocks is characteristic, as HSP typically affects the lower extremities and buttocks. 3, 4

Why Other Diagnoses Are Incorrect

• Churg-Strauss syndrome (A): This ANCA-associated vasculitis presents with asthma, eosinophilia, and pulmonary infiltrates—none of which are present in this case. 5 It is also extremely rare in adolescents. 5

• Hemolytic uremic syndrome (B): HUS presents with the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. 3 This patient has normal kidneys on ultrasound and no mention of anemia or thrombocytopenia. The palpable purpura is not consistent with HUS, which typically causes petechiae from thrombocytopenia rather than palpable purpura from vasculitis. 3

• Thrombotic thrombocytopenic purpura (D): TTP presents with the pentad of fever, microangiopathic hemolytic anemia, thrombocytopenia, renal dysfunction, and neurological symptoms. 3 The purpura in TTP would be petechial (non-palpable) due to thrombocytopenia, not the palpable purpura seen in vasculitis. 3 TTP is also exceedingly rare in adolescents. 3

Immediate Management Steps

Essential initial workup includes:

• Urinalysis with microscopy: Look specifically for red blood cell casts and dysmorphic RBCs (indicate glomerular involvement), and quantify proteinuria using spot urine protein-to-creatinine ratio. 2, 1

• Basic metabolic panel: Including BUN, serum creatinine, and complete blood count with platelets to assess renal function and rule out thrombocytopenia. 2, 1

• Blood pressure measurement: Essential as hypertension indicates more severe renal involvement. 2, 1

• Renal ultrasound: Already performed and normal, which is reassuring but does not exclude HSP nephritis. 2

Critical Management Considerations

Do NOT use corticosteroids prophylactically to prevent nephritis—moderate quality evidence (Level 1B) shows no benefit in preventing nephritis or reducing risk of severe persistent nephritis. 5, 1, 2 This is a common pitfall, as early steroid treatment does not reduce the incidence and severity of nephropathy in children with HSP. 3

For pain control, avoid NSAIDs (like ketorolac/Toradol) in HSP patients with renal involvement, as they can cause acute kidney injury. 1, 2 Use acetaminophen as first-line analgesic instead. 1

Treatment algorithm based on renal involvement:

• Moderate proteinuria (0.5-1 g/day per 1.73 m²): Start ACE inhibitor or ARB therapy, targeting proteinuria to <1 g/day/1.73 m². 5, 1, 2, 3

• Persistent proteinuria >1 g/day per 1.73 m² after ACE-I/ARB trial and GFR >50 ml/min per 1.73 m²: Consider 6-month course of corticosteroid therapy. 5, 1, 2

• Crescentic HSP with nephrotic syndrome and/or deteriorating kidney function: Treat with high-dose intravenous methylprednisolone plus cyclophosphamide, following the same protocol as crescentic IgA nephropathy. 5, 1, 2

Follow-Up Protocol

Follow-up should continue for at least 6 months and include regular urine testing for proteinuria and hematuria plus blood pressure measurement. 6, 7 Most cases are self-limited with an average duration of 4 weeks, but renal involvement is the most important prognostic factor determining long-term morbidity. 3, 6, 7