What is the best course of action for a 6-year-old patient who presents with thigh pain, following a recent illness with fever and rhinorrhea one week prior?

Urgent Evaluation for Septic Arthritis or Osteomyelitis

This 6-year-old with thigh pain following a recent febrile illness requires immediate evaluation for septic arthritis or osteomyelitis, as post-viral bacteremia can seed joints and bones, causing permanent damage within hours if untreated. 1, 2, 3

Critical Red Flags Present in This Case

• Recent fever with rhinorrhea followed by new-onset thigh pain strongly suggests post-viral bacteremia with hematogenous spread to bone or joint 4, 5
• Anterior bilateral proximal thigh pain and tenderness is a diagnostically useful manifestation of bacteremia and should prompt immediate blood cultures 5
• The timing (one week post-viral illness) is classic for secondary bacterial infection, as viral upper respiratory infections predispose to subsequent bacterial seeding 4, 6

Immediate Diagnostic Workup Required

Obtain the following immediately:

• Blood cultures before any antibiotics 3
• Complete blood count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) 4
  • ESR >36 mm/hour and CRP >60 mg/L are associated with osteomyelitis or septic arthritis 4
• Urgent hip ultrasound bilaterally to assess for joint effusion, even though pain is in the thigh, as hip pathology presents as thigh pain in 30% of pediatric cases 1, 2, 3

Initial imaging must include:

• Plain radiographs of bilateral tibia/fibula and hip/pelvis 2
• If hip effusion is detected on ultrasound, perform immediate ultrasound-guided aspiration for cell count, Gram stain, and culture 1, 3

Physical Examination Priorities

Assess specifically for:

• Ability to bear weight - inability to bear weight is significantly associated with bacterial infection (osteomyelitis, septic arthritis, or intramuscular abscess) 4
• The "three As" - anxiety, agitation, and high analgesic requirement suggest serious pathology 2
• Focal tenderness along the entire lower extremity, including hip, femur, and knee 2
• Signs of septic arthritis: refusal to move the hip, pain with passive range of motion 1

Differential Diagnosis by Priority

1. Septic arthritis of the hip - most common cause of acute severe monoarticular pain in children, typically from hematogenous spread of Staphylococcus aureus 1
2. Osteomyelitis of the femur or pelvis - consider if no hip effusion but elevated inflammatory markers 1, 4
3. Pyomyositis (obturator internus or iliopsoas muscle abscess) - rare but presents with thigh pain, fever, and positive blood cultures for S. aureus 7
4. Transient synovitis - less likely given recent fever, but cannot be excluded without aspiration if effusion present 3

Management Algorithm

If fever or elevated inflammatory markers present:

• Start empiric IV vancomycin immediately after obtaining blood cultures to cover MRSA 3
• Perform urgent hip ultrasound 1, 3
• If effusion present, perform immediate aspiration 1, 3
• Admit for IV antibiotics and surgical consultation 3

If ultrasound shows no hip effusion but high clinical suspicion:

• Consider MRI of pelvis to evaluate for osteomyelitis or pyomyositis 1, 3
• Up to 30% of children with septic arthritis have concomitant osteomyelitis requiring longer treatment 3
• MRI has high sensitivity and specificity for musculoskeletal infection 1

If initial radiographs are normal but symptoms persist:

• Repeat radiographs in 7-10 days, as approximately 10% of fractures (including toddler's fractures) only appear on follow-up imaging 2, 3
• Consider MRI if symptoms worsen or red flags develop 2

Critical Pitfalls to Avoid

• Never assume normal radiographs exclude serious pathology - septic arthritis and early osteomyelitis have normal plain films 1
• Never focus only on the thigh - examine and image the hip, as 30% of hip pathology presents as referred thigh or knee pain 2, 3
• Never delay evaluation when fever accompanies inability to bear weight - septic arthritis causes permanent joint damage within hours 1, 2, 3
• Never discharge without close follow-up if infection cannot be definitively excluded, as post-viral bacteremia can present subtly 4, 5

Disposition Decision

Admit immediately if:

• Confirmed or suspected septic arthritis, osteomyelitis, or bacteremia 3
• Unable to exclude serious infection despite workup 3
• Inability to bear weight with elevated inflammatory markers 4

Duration of antibiotic therapy if infection confirmed:

• Uncomplicated osteomyelitis or septic arthritis: 3-3.5 weeks total (IV followed by oral) 8
• Concomitant osteomyelitis with septic arthritis: longer duration required 3, 8