What is the most likely diagnosis in a patient with fever, urinary tract infection, knee effusion suggestive of septic arthritis, a cracked (strawberry) tongue, diarrhea, and a recent tattoo?

Kawasaki Disease

The constellation of fever, urinary tract infection, knee effusion (septic arthritis), strawberry/cracked tongue, diarrhea, and recent tattoo is most consistent with Kawasaki disease, though septic arthritis must be immediately excluded through joint aspiration before attributing all findings to a single vasculitic process. 1, 2

Immediate Diagnostic Priorities

Rule Out Septic Arthritis First

• In any febrile patient with knee effusion, septic arthritis must be presumed until proven otherwise because bacterial proliferation can cause irreversible cartilage damage within hours to days. 1, 2, 3
• Obtain blood cultures immediately, then perform joint aspiration before starting antibiotics. 1, 2
• Synovial fluid analysis with cell count (≥50,000 cells/mm³ suggests septic arthritis), Gram stain (sensitivity 76%, specificity 96%), and culture is definitive. 1, 2
• Start empiric IV vancomycin 15 mg/kg every 6 hours immediately after cultures to cover MRSA, the most common pathogen across all ages. 1, 2, 3

Initial Imaging Algorithm

• Obtain knee radiographs first to exclude fractures, tumors, and identify joint effusion, though radiographs are normal in early infection (<14 days) in approximately two-thirds of cases. 4, 1, 2
• Ultrasound can detect effusions as small as 1 mL and guide aspiration, but may yield false-negative results if performed within 24 hours of symptom onset. 1, 2
• MRI with contrast (sensitivity 82-100%, specificity 75-96%) is indicated if clinical suspicion remains high despite negative aspiration, or if concurrent osteomyelitis is suspected (present in >50% of pediatric septic arthritis cases). 1, 2

Kawasaki Disease Diagnostic Considerations

Classic Clinical Features Present

• Strawberry/cracked tongue is a hallmark mucosal finding in Kawasaki disease, representing one of the five principal diagnostic criteria. 1
• Fever lasting ≥5 days is the primary criterion. 4
• Sterile pyuria mimicking UTI occurs in 60-80% of Kawasaki disease cases due to urethritis from systemic vasculitis, not true bacterial infection. 4
• Arthritis/arthralgia occurs in 15-50% of cases, typically affecting large joints including knees. 1
• Diarrhea is a common gastrointestinal manifestation. 4

Tattoo as Potential Trigger

• Recent tattoo represents a potential infectious or inflammatory trigger, as Kawasaki disease can be precipitated by various antigenic stimuli in genetically susceptible individuals. 4
• However, the tattoo also raises concern for direct bacterial inoculation causing true septic arthritis, making joint aspiration mandatory. 1, 2, 3

Diagnostic Algorithm

Step 1: Exclude Bacterial Septic Arthritis

1. Perform joint aspiration with synovial fluid analysis (cell count, Gram stain, culture, crystal analysis). 1, 2
2. If synovial WBC ≥50,000 cells/mm³ or positive Gram stain, proceed with surgical drainage and continue antibiotics. 1, 2
3. If synovial fluid is inflammatory but culture-negative and clinical response to antibiotics is poor after 48-72 hours, consider MRI to evaluate for concurrent osteomyelitis or alternative diagnosis. 1, 2

Step 2: Evaluate for Kawasaki Disease

• Assess for five principal criteria: (1) fever ≥5 days, (2) bilateral conjunctival injection, (3) oral mucosal changes (strawberry tongue, cracked lips, pharyngeal erythema), (4) polymorphous rash, (5) extremity changes (erythema, edema, desquamation), (6) cervical lymphadenopathy >1.5 cm. 4
• Diagnosis requires fever plus ≥4 of 5 principal criteria, or fever plus <4 criteria with coronary artery abnormalities on echocardiography (incomplete Kawasaki disease). 4
• Obtain echocardiography urgently to assess for coronary artery dilation or aneurysms, the most serious complication. 4

Step 3: Laboratory Evaluation

• Elevated ESR and CRP support both septic arthritis and Kawasaki disease, so cannot differentiate. 4, 1, 2
• Urinalysis showing sterile pyuria (WBCs without bacteria on culture) strongly suggests Kawasaki disease rather than true UTI. 4
• Thrombocytosis (platelets >450,000/mm³) typically appears in week 2-3 of Kawasaki disease. 4
• Hypoalbuminemia, elevated transaminases, and anemia are common in Kawasaki disease. 4

Critical Management Decisions

If Septic Arthritis is Confirmed

• Continue IV vancomycin and perform surgical drainage (arthroscopy or arthrotomy). 1, 2, 5
• Transition to oral antibiotics after 2-4 days if clinically improving, afebrile, and tolerating oral intake; total duration 3-4 weeks (or 2 weeks after drainage in select cases). 1, 2
• Longer treatment required if concurrent osteomyelitis is present. 1, 2

If Kawasaki Disease is Diagnosed

• Administer high-dose IVIG 2 g/kg as a single infusion plus high-dose aspirin 80-100 mg/kg/day divided every 6 hours until afebrile for 48-72 hours, then low-dose aspirin 3-5 mg/kg/day for 6-8 weeks. 4
• Treatment within 10 days of fever onset reduces coronary artery aneurysm risk from 25% to <5%. 4
• Repeat echocardiography at 2 weeks and 6-8 weeks after diagnosis. 4

If Both Conditions Coexist

• Treat septic arthritis with surgical drainage and antibiotics while simultaneously treating Kawasaki disease with IVIG and aspirin. 1, 2
• IVIG does not interfere with antibiotic efficacy for bacterial infections. 4

Key Pitfalls to Avoid

• Do not assume all findings are due to Kawasaki disease without first excluding septic arthritis through joint aspiration, as delayed treatment of bacterial arthritis causes irreversible cartilage destruction. 1, 2
• Do not attribute pyuria to UTI without urine culture; sterile pyuria is a common Kawasaki disease finding and does not require antibiotics. 4
• Do not delay IVIG administration beyond 10 days of fever onset if Kawasaki disease is diagnosed, as this increases coronary artery complication risk. 4
• Do not stop aspirin therapy abruptly; continue low-dose aspirin for 6-8 weeks even after clinical resolution. 4
• If joint aspiration is negative but clinical suspicion for infection remains high, consider percutaneous bone biopsy to evaluate for concurrent osteomyelitis. 1, 2
• Always perform crystal analysis on synovial fluid, as crystal arthropathies can coexist with septic arthritis in 73% of reported cases. 2