What is the most likely diagnosis and appropriate management for a male child presenting with a urinary tract infection, knee effusion, conjunctivitis, and a recent tattoo?

Reactive Arthritis (Reiter Syndrome)

This clinical presentation—urinary tract infection, knee effusion, conjunctivitis, and recent tattoo in a male child—is most consistent with reactive arthritis (formerly Reiter syndrome), a post-infectious inflammatory condition that requires prompt recognition and multidisciplinary management.

Clinical Diagnosis

The classic triad of reactive arthritis consists of:

• Arthritis (knee effusion in this case) 1
• Conjunctivitis 1, 2
• Urethritis/UTI (genitourinary symptoms) 1, 2

• This syndrome typically occurs after a genitourinary or gastrointestinal infection, and the tattoo may represent a potential infectious trigger or site of bacterial entry 1, 2
• Reactive arthritis is more common in males and has been reported in children, though it predominantly affects young adults 1
• The condition is strongly associated with HLA-B27 positivity, which may be present in this patient 2

Additional Clinical Features to Assess

Beyond the classic triad, evaluate for:

• Mucocutaneous lesions: keratoderma blennorrhagicum (pustular lesions on palms/soles), circinate balanitis, oral ulcerations 1, 2
• Musculoskeletal findings: dactylitis ("sausage digits"), Achilles tendonitis, sacroiliitis, lower back pain 2
• Systemic symptoms: fever, malaise, elevated inflammatory markers (ESR, CRP) 2

Immediate Diagnostic Workup

Essential laboratory and microbiological studies:

• Urine culture via catheterization or suprapubic aspiration (not bag collection) to confirm UTI and identify pathogen 3
• Urethral/cervical swab for Chlamydia trachomatis PCR, as this is a common trigger 2
• Stool culture if gastrointestinal symptoms present (to exclude enteric pathogens like Salmonella, Shigella, Campylobacter) 1
• Joint aspiration of knee effusion to exclude septic arthritis (synovial fluid analysis: cell count, Gram stain, culture) 2
• HLA-B27 testing to support diagnosis, though not required for management 2
• Inflammatory markers: CBC, ESR, CRP, rheumatoid factor (typically negative in reactive arthritis) 2

Management of UTI Component

While evaluating for reactive arthritis, treat the UTI appropriately:

• Obtain urine culture before starting antibiotics to guide subsequent therapy 3, 4
• Initiate empiric oral antibiotics for 7-14 days: amoxicillin-clavulanate, cephalosporins (cefixime, cephalexin), or trimethoprim-sulfamethoxazole if local resistance <10% 4, 5
• Consider parenteral therapy (ceftriaxone 50 mg/kg IV/IM daily) if the child appears toxic, cannot tolerate oral intake, or has uncertain compliance 4, 5
• Adjust antibiotics based on culture and sensitivity results when available 4

Management of Reactive Arthritis

Treatment requires a multidisciplinary approach:

• NSAIDs (first-line for arthritis and inflammatory symptoms) 2
• Systemic corticosteroids may be necessary for severe or refractory cases 2
• Antibiotics to eradicate the triggering infection (Chlamydia requires doxycycline or azithromycin; adjust based on culture results) 2
• Physical therapy for joint symptoms and to maintain mobility 2

Imaging Recommendations

For the UTI component:

• Renal and bladder ultrasound (RBUS) is recommended for febrile UTI in children <2 years to detect anatomic abnormalities 3, 4, 6
• Voiding cystourethrography (VCUG) is NOT routinely indicated after first UTI but should be performed if RBUS shows abnormalities or after a second febrile UTI 3, 4

For reactive arthritis:

• Plain radiographs of affected joints (knee, sacroiliac joints if back pain present) to assess for sacroiliitis or erosive changes 2
• CT or MRI may be needed if imaging shows sacroiliitis or spondylitis 2

Follow-Up and Prognosis

• Clinical reassessment within 24-48 hours to confirm fever resolution and response to antibiotics 4
• Monitor for chronic sequelae: 15-20% of patients with reactive arthritis develop severe chronic complications, including chronic arthritis, uveitis, or cardiac involvement 1
• Refer to pediatric rheumatology for ongoing management of reactive arthritis 2
• Instruct parents to seek prompt evaluation for any future febrile illnesses, as recurrent UTI risk is significant 4

Critical Pitfalls to Avoid

• Do not delay antibiotic treatment for the UTI while awaiting culture results, as early treatment reduces renal scarring risk 4
• Do not use bag collection for urine culture, as false-positive rates are unacceptably high (85%) 3
• Do not miss septic arthritis: joint aspiration is mandatory to exclude bacterial joint infection, which requires urgent surgical drainage 2
• Do not overlook Chlamydia testing: this is a common trigger in reactive arthritis and requires specific antibiotic therapy 2
• Do not assume the tattoo is unrelated: it may represent the infectious trigger or a site of bacterial entry requiring evaluation 1, 2