What is the most likely diagnosis in a 7‑year‑old boy with a 10‑day history of arthralgias of the knees and ankles, a 7‑day non‑pruritic rash that began as urticaria and progressed to palpable purpura on the legs, buttocks, and posterior arms, intermittent colicky abdominal pain, normal CBC, comprehensive metabolic panel, coagulation studies, and a negative urinalysis, after an upper‑respiratory infection three weeks ago?

IgA Vasculitis (Henoch-Schönlein Purpura) is the Most Likely Diagnosis

The clinical presentation of palpable purpura on the lower extremities and buttocks, arthralgias of the knees and ankles, colicky abdominal pain, normal platelet count, and preceding upper respiratory infection in a 7-year-old boy is classic for IgA vasculitis (Henoch-Schönlein purpura). 1, 2

Why IgA Vasculitis is the Correct Answer

Classic Clinical Triad Present

• Palpable purpura (not just petechiae) on the lower extremities, buttocks, and posterior arms is the hallmark finding, occurring in 95% of IgA vasculitis cases 2, 3
• Arthritis/arthralgia affecting the knees and ankles with periarticular swelling but no effusion is characteristic 1, 4
• Abdominal pain (colicky, without bloody stools in this case) completes the classic triad 4, 5

Key Diagnostic Features Supporting IgA Vasculitis

• Age: Peak incidence is 4-11 years old, making this 7-year-old in the typical age range 1, 3
• Preceding viral illness: Upper respiratory infection 3 weeks prior is a common trigger, often associated with group A streptococci or other infectious agents 4, 5
• Rash evolution: The progression from urticarial lesions to palpable purpura over several days is characteristic of IgA vasculitis 1, 2
• Normal laboratory values: Normal CBC (including platelet count), coagulation studies, and urinalysis at presentation are typical, as IgA vasculitis is a clinical diagnosis 2, 6

Why Other Diagnoses Are Excluded

Meningococcemia (Option d) - Excluded

• Appears nontoxic with normal vital signs, which is incompatible with meningococcemia 1
• Meningococcemia presents with acute onset of fever, septic shock, and rapid deterioration within hours to days 1
• The 10-day history of symptoms with stable condition rules out this life-threatening diagnosis 1

Rocky Mountain Spotted Fever (Option e) - Excluded

• No fever documented on examination, whereas RMSF typically presents with fever >38.5°C (103.5°F) 7
• RMSF has a rapid, fulminant course with 50% of deaths occurring within 9 days of onset 7
• The rash in RMSF classically starts on wrists and ankles then spreads centrally, not primarily on buttocks and posterior arms 7
• While 40% of RMSF patients may not report tick exposure, the hiking trip was 3 weeks ago (before symptom onset), making the timeline inconsistent 7, 1

Idiopathic Thrombocytopenic Purpura (Option b) - Excluded

• Normal platelet count definitively rules out ITP 8
• ITP presents with non-palpable petechiae and ecchymoses, not palpable purpura 8
• Joint involvement is not a feature of ITP 8

Hemolytic Uremic Syndrome (Option a) - Excluded

• Normal comprehensive metabolic panel excludes renal dysfunction 1
• HUS typically presents with bloody diarrhea, acute renal failure, and microangiopathic hemolytic anemia 1
• The absence of vomiting, bloody stools, and normal laboratory values make HUS extremely unlikely 1

Critical Management Considerations

Immediate Assessment Required

• Exclude meningococcemia first in any child with purpuric rash, even with normal vital signs, as this is the most critical life-threatening differential 1
• Assess for hemodynamic instability within 1 hour of presentation 1

Renal Monitoring is Essential

• Urinalysis should be repeated even though initially negative, as IgA nephritis can develop later and is the most serious long-term complication 2, 3
• Follow-up for 6-12 months is required to monitor for renal involvement, which accounts for 1-2% of childhood end-stage kidney disease 3
• Patients who develop nephrotic/nephritic syndrome or significant persistent proteinuria require renal biopsy 3

Treatment Approach

• Supportive care is the primary management, as IgA vasculitis spontaneously resolves in 94% of children 2
• Corticosteroids should not be used prophylactically to prevent complications, as systematic reviews show no benefit 2
• Steroids may be indicated for severe gastrointestinal symptoms or renal involvement if it develops 2, 3

Common Pitfalls to Avoid

• Do not wait for laboratory confirmation before making the diagnosis, as IgA vasculitis is a clinical diagnosis 2, 6
• Do not assume the disease is benign without appropriate follow-up, as renal involvement can persist and relapse years later 2, 3
• Do not exclude IgA vasculitis if the complete triad is not present initially, as any component may be absent at presentation 4, 5