✅ Model Answer:
• Diagnosis: Reactive arthritis (Reiter syndrome) — post-enteric arthritis (Campylobacter), acute asymmetric arthritis of the right knee, elevated ESR/CRP, normal WBC. The classic triad (arthritis, conjunctivitis, urethritis) may not all be present initially. HLA-B27 is positive in most cases.
• Any other test: HLA-B27 (positive), synovial fluid analysis (exclude septic arthritis — WBC <50,000, negative Gram stain/culture), Chlamydia testing (if sexually active), HIV/hepatitis screen, ophthalmology exam (for conjunctivitis/uveitis).
• What to do next: NSAIDs (ibuprofen or naproxen) for arthritis. Treat underlying infection (Campylobacter is usually self-limited, but antibiotics may be given if persistent). If severe or chronic, consider sulfasalazine or TNF inhibitor.
• Follow-up plan: Monitor for persistent arthritis (>6 months) → may evolve into enthesitis-related arthritis (ERA). Treat conjunctivitis with topical steroids if severe. Resolves in weeks to months. Screen for uveitis. Rheumatology follow-up.

✅ Model Answer:
• Reactive arthritis (Reiter syndrome) is an inflammatory arthritis that develops after an infection, typically 1-4 weeks after the initial infection.
• Triggers:
- Enteric infections: Campylobacter, Salmonella, Shigella, Yersinia, Clostridium difficile.
- Genitourinary infections: Chlamydia trachomatis, Ureaplasma urealyticum.
- Other: Respiratory infections (rare).
• Pathophysiology: Immune-mediated (cross-reactivity between bacterial antigens and joint tissues).
• Age: Most common in adolescents and young adults.
• Gender: More common in males (especially if post-venereal).
• HLA-B27: Associated with HLA-B27 positivity (50-80% of cases).
• Classic triad: Arthritis + conjunctivitis + urethritis (not always present; "incomplete" Reiter syndrome is common).

✅ Model Answer:
• Arthritis:
- Asymmetric oligoarthritis: Usually affects the lower extremities (knees, ankles, feet).
- Acute onset: Joint swelling, pain, stiffness (often worse in the morning).
- Dactylitis (sausage digits): Swelling of the entire finger or toe.
- Enthesitis: Inflammation at tendon insertions (Achilles tendon, plantar fascia).
- Back pain: Sacroiliitis (in HLA-B27 positive patients).
• Extra-articular manifestations:
- Conjunctivitis: Non-gonococcal, often bilateral (30-50% of cases).
- Urethritis: Dysuria, urethral discharge (especially in males).
- Mucocutaneous: Oral ulcers, balanitis circinata (painless penile ulcers), keratoderma blennorrhagica (papulosquamous skin lesions on palms and soles).
- Uveitis (anterior): Painful red eye, photophobia.
- Fever, malaise, weight loss.
• Onset: 1-4 weeks after the precipitating infection.

✅ Model Answer:
• HLA-B27: A major histocompatibility complex (MHC) class I antigen, strongly associated with spondyloarthropathies.
• Prevalence: 50-80% of patients with reactive arthritis are HLA-B27 positive (compared to 8% of the general population).
• Prognostic significance:
- HLA-B27 positive: Higher risk of chronic arthritis, sacroiliitis, and ankylosing spondylitis.
- HLA-B27 negative: Usually milder, self-limited disease.
- Associated with more severe: Extra-articular manifestations (uveitis, enthesitis).
• Mechanism: HLA-B27 may present bacterial peptides to T-cells, triggering an autoimmune response (molecular mimicry).
• Testing: HLA-B27 testing is recommended in patients with reactive arthritis (especially if symptoms persist >1 month).
• Implication: If HLA-B27 positive, the patient is at increased risk for ankylosing spondylitis and other seronegative spondyloarthropathies.

✅ Model Answer:
• Reactive arthritis:
- Onset: Subacute (1-4 weeks after infection).
- Fever: Low-grade or absent.
- WBC: Normal or mildly elevated.
- ESR/CRP: Elevated (but not as high as septic arthritis).
- Joint: Multiple joints (oligoarthritis), asymmetric.
- Synovial fluid: WBC <50,000, negative Gram stain/culture.
- Systemic symptoms: Conjunctivitis, urethritis, skin lesions (if present).
- History: Recent infection (gastrointestinal or genitourinary).
• Septic arthritis:
- Onset: Acute (hours to days).
- Fever: High (>38.5°C).
- WBC: Elevated (>12,000/µL).
- ESR/CRP: Highly elevated.
- Joint: Monoarticular (most often hip or knee).
- Synovial fluid: WBC >50,000, positive Gram stain/culture.
- Systemic symptoms: Usually none (unless bacteremia).
- History: No preceding infection (usually).
• If in doubt: Arthrocentesis is the gold standard to differentiate.

✅ Model Answer:
• NSAIDs (first-line):
- Ibuprofen: 30-40 mg/kg/day (max 2.4 g/day).
- Naproxen: 10-20 mg/kg/day.
- Indomethacin: 1-2 mg/kg/day (for severe cases).
- Duration: Continue until symptoms resolve (usually 4-6 weeks).
• Treat underlying infection:
- Campylobacter: Usually self-limited; antibiotics (azithromycin) if persistent.
- Chlamydia: Azithromycin (1 g single dose) or doxycycline (if sexually active).
- Salmonella/Shigella: Antibiotics if severe or immunocompromised.
• If persistent or severe (>6 months):
- Sulfasalazine: 50 mg/kg/day (for chronic arthritis).
- Methotrexate: 10-15 mg/m²/week (for refractory arthritis).
- TNF inhibitors: Etanercept, adalimumab (for refractory disease).
- Corticosteroids: Intra-articular injections (for monoarticular arthritis).
- Ophthalmology: If uveitis or conjunctivitis develops.

✅ Model Answer:
• Complications:
- Chronic arthritis: 20-30% of patients develop persistent arthritis (>6 months).
- Enthesitis-related arthritis (ERA): May evolve into a chronic spondyloarthropathy.
- Ankylosing spondylitis: In HLA-B27 positive patients (especially with sacroiliitis).
- Uveitis: Anterior uveitis (can cause blindness if untreated).
- Conjunctivitis: Usually self-limited, but may recur.
- Mucocutaneous: Balanitis circinata, keratoderma blennorrhagica.
- Cardiac: Aortic regurgitation (rare, but can occur in chronic cases).
- Joint destruction: Rarely, can cause permanent joint damage.
- Functional impairment: If chronic arthritis develops.
- Relapse: May recur with subsequent infections.
- Death: Very rare.

✅ Model Answer:
• Prognosis:
- Good: Most children recover completely within 3-6 months.
- Recurrence: Possible if a new infection occurs (especially with HLA-B27 positive).
- Chronic disease: 20-30% of patients may develop persistent arthritis (>6 months).
- HLA-B27 positive: Higher risk of chronic arthritis, sacroiliitis, and ankylosing spondylitis.
- Functional outcome: Good with appropriate treatment; joint damage is rare.
- Life expectancy: Normal.
• Long-term follow-up:
- Rheumatology: Follow-up if symptoms persist >6 weeks.
- Ophthalmology: Screen for uveitis if eye symptoms develop.
- Physical therapy: To maintain joint function and strength.
- Education: Advise about recurrence with future infections.
- HLA-B27 counseling: If positive, discuss risk of ankylosing spondylitis.
- Activity modification: Avoid high-impact activities during active arthritis.
- Monitor for chronic disease: If symptoms persist >6 months, consider DMARDs (methotrexate, sulfasalazine).