What is the most likely diagnosis for a 2-year-old boy presenting with refusal to walk, fever (hyperthermia), hip pain, limited mobility, and an elevated white blood cell (WBC) count, with vital signs indicating hypotension, tachycardia, and tachypnea?

Septic Arthritis of the Hip

This 2-year-old boy most likely has septic arthritis of the left hip, which is an orthopedic emergency requiring immediate joint aspiration and empiric antibiotic therapy.

Clinical Reasoning

The constellation of findings strongly points to septic arthritis rather than the other differential diagnoses:

• High fever (39.2°C) combined with refusal to walk and hip pain is the critical distinguishing feature 1
• Classic positioning: Hip held in flexion, abduction, and external rotation (position of maximal joint capsule volume to minimize pain) 1
• Pain with passive movement indicates true intra-articular pathology 1
• Elevated WBC count (13,000 cells/mm³) meets one of the validated Kocher criteria (WBC ≥12,000 cells/mm³) 1
• Systemic signs: Tachycardia (140 bpm) and tachypnea (24/min) out of proportion to fever suggest systemic inflammatory response 2

Why Not the Other Diagnoses?

Transient synovitis is the most common cause of acute hip pain in children aged 2 years 3, but it is essentially ruled out here by:

• The presence of high fever (transient synovitis is typically afebrile or low-grade fever) 1, 3
• Systemic toxicity with tachycardia and possible hypotension 4
• Fever is the main distinguishing factor between harmless conditions and life-threatening septic arthritis 3

Legg-Calvé-Perthes disease:

• Typically presents in children aged 4-8 years, not 2 years 3
• Insidious onset over weeks to months, not acute 2-day presentation
• No fever or systemic signs

Slipped capital femoral epiphysis (SCFE):

• Occurs in older children (typically 10-16 years) during growth spurts 3
• No fever or elevated WBC count
• Wrong age group entirely

Diagnostic Approach

Immediate workup required 1:

• Ultrasound of the hip to confirm joint effusion and guide aspiration 1
• Joint aspiration for synovial fluid analysis (cell count, Gram stain, culture) before antibiotics 2, 1
• Additional labs: ESR and CRP (ESR ≥40 mm/hr and CRP >2.0 mg/dL are validated predictors) 1
• Blood cultures before antibiotic administration 2
• Plain radiographs to rule out other pathology, though sensitivity for septic arthritis is low 1

Critical Kocher Criteria Assessment

The validated Kocher criteria for predicting septic arthritis include 1:

1. Fever - Present (39.2°C) ✓
2. Non-weight-bearing - Present (refuses to walk) ✓
3. ESR ≥40 mm/hr - Not yet obtained
4. WBC ≥12,000 cells/mm³ - Present (13,000) ✓

With 3 of 4 criteria already met, the probability of septic arthritis is very high 1. The combination of all four criteria has the highest predictive value 1.

Pathophysiology and Urgency

Why this is an emergency 2, 5:

• Bacterial proliferation causes irreversible cartilage damage within hours to days through direct bacterial toxins and inflammatory mediators 2, 5
• Staphylococcus aureus (most common pathogen in this age group) produces toxins and proteolytic enzymes that rapidly destroy cartilage 2
• In children under 4 years, Kingella kingae should also be strongly considered 2, 1
• Concomitant osteomyelitis occurs in >50% of pediatric septic arthritis cases 2, 1

Management Priorities

Immediate interventions:

• Orthopedic consultation for urgent surgical drainage 2
• Empiric antibiotics after cultures obtained: vancomycin (to cover MRSA) plus ceftriaxone or cefotaxime 2
• Fluid resuscitation given borderline blood pressure (90/40 mmHg may be low for a 2-year-old) 4

Common Pitfalls to Avoid

• Do not delay aspiration waiting for advanced imaging like MRI 1
• Do not assume transient synovitis in the presence of fever >38.5°C 1, 3
• Do not miss concomitant osteomyelitis: MRI detects osteomyelitis in approximately 50% of children with clinically suspected septic arthritis 1
• Consider alternative diagnoses if no effusion on ultrasound: pyomyositis of hip muscles (obturator internus, iliopsoas) can mimic septic arthritis 6, 4
• Negative synovial fluid cultures occur in approximately 20% of cases despite true infection 2

The clinical presentation overwhelmingly favors septic arthritis, and any delay in diagnosis and treatment risks permanent joint damage and systemic complications 2, 5.