In a child with a 7‑week history of left knee pain, morning stiffness, warmth, effusion, reduced range of motion, no fever or systemic symptoms, and negative blood and stool cultures, what is the most likely diagnosis?

Diagnosis: Oligoarticular Juvenile Idiopathic Arthritis (Option A)

The most likely diagnosis is oligoarticular juvenile idiopathic arthritis (oligoJIA), given the 7-week duration of monoarticular knee involvement with morning stiffness, absence of systemic symptoms, and negative cultures. 1

Clinical Reasoning

Why Oligoarticular JIA is the Correct Answer

• Oligoarticular JIA is the most common form of chronic arthritis in children, accounting for the majority of JIA cases in both Europe and North America 2, 3
• The knee is the most frequently affected joint in oligoarticular JIA, with a strong predilection for large joints 4
• Morning stiffness is a hallmark feature of inflammatory arthritis, distinguishing it from mechanical or infectious causes 1
• The 7-week duration meets the diagnostic requirement that arthritis must persist for at least 6 weeks to diagnose JIA 1
• Absence of fever and systemic symptoms effectively rules out systemic-onset JIA, which characteristically presents with quotidian fevers, evanescent rash, and systemic manifestations 5
• Negative blood and stool cultures make infectious etiologies highly unlikely 6

Why Other Options Are Incorrect

Septic Arthritis (Option B) is excluded because:

• Septic arthritis is an orthopedic emergency that typically presents acutely, not over 7 weeks 6, 7
• The Kocher criteria help distinguish septic arthritis from other conditions: fever >101.3°F, ESR ≥40 mm/hour, WBC ≥12,000 cells/mm³, and inability to bear weight 6
• This patient lacks fever and has negative cultures, making septic arthritis extremely unlikely 6
• Septic arthritis would cause rapid cartilage damage if untreated for 7 weeks, which is inconsistent with the clinical presentation 6, 7

Reactive Arthritis (Option C) is less likely because:

• Reactive arthritis typically follows a gastrointestinal or genitourinary infection by 1-4 weeks 6
• The 7-week duration without resolution suggests chronic inflammatory arthritis rather than post-infectious reactive arthritis 6
• Negative stool cultures argue against a preceding enteric infection 6

Hemophilia A (Option D) is excluded because:

• Hemophilic arthropathy presents with recurrent hemarthroses following trauma, not isolated morning stiffness 1
• There is no mention of bleeding history, trauma, or family history of bleeding disorders
• Morning stiffness and warmth are inflammatory features, not typical of hemarthrosis

Diagnostic Approach

Essential Clinical Features to Confirm

• Joint involvement pattern: Oligoarticular JIA affects 1-4 joints in the first 6 months of disease 1, 3
• Morning stiffness duration: Typically lasts >15 minutes and improves with activity 1
• Age considerations: Peak incidence is 1-5 years with a second peak in early adolescence 5
• Gender: More common in females (approximately 65% of cases) 4

Laboratory Investigations to Order

• Complete blood count with differential: To assess for leukocytosis (suggests infection) versus normal or mild elevation (consistent with JIA) 1
• Inflammatory markers (ESR and CRP): Elevated in approximately 50% of oligoarticular JIA cases at presentation 1, 4
• Antinuclear antibody (ANA): Positive in approximately 40% of oligoarticular JIA patients and identifies those at higher risk for uveitis 1, 4
• Rheumatoid factor: Typically negative in oligoarticular JIA; positivity would suggest polyarticular RF-positive JIA 1

Imaging Considerations

• Ultrasound or MRI can detect joint effusion, synovial thickening, and early inflammatory changes when clinical examination is equivocal 1
• Plain radiographs are typically normal early in disease but may show soft tissue swelling or joint effusion 6

Critical Management Points

Immediate Actions

• Refer to pediatric rheumatology for definitive diagnosis and treatment planning 2
• Screen for uveitis: All children with oligoarticular JIA require ophthalmologic screening, as chronic anterior uveitis occurs in 20-30% of ANA-positive patients and can be asymptomatic 1
• Initiate NSAIDs (naproxen is commonly used) for symptomatic relief while awaiting rheumatology evaluation 4

Treatment Escalation Strategy

• Intra-articular corticosteroid injections are highly effective for oligoarticular disease and used in approximately 30% of patients 4
• Methotrexate is indicated if NSAIDs and intra-articular steroids fail to control disease, used in approximately 38% of oligoarticular JIA patients 8, 4
• Biologic DMARDs (such as adalimumab) are reserved for refractory cases, required in approximately 13% of patients 4

Common Pitfalls to Avoid

• Do not delay diagnosis waiting for multiple joints to become involved: Oligoarticular JIA can remain monoarticular (persistent oligoarticular) or progress to involve more joints (extended oligoarticular) 8, 3
• Do not assume absence of elevated inflammatory markers excludes JIA: Approximately 50% of oligoarticular JIA patients have normal ESR at presentation 4
• Do not miss uveitis screening: This is the most serious complication of oligoarticular JIA and can lead to blindness if untreated 1
• Do not confuse the 7-week duration with acute septic arthritis: The subacute presentation over weeks with negative cultures clearly distinguishes chronic inflammatory arthritis from acute infection 6, 7