In a neonate with septic arthritis of the hip, the most common causative organism and the clinical feature that distinguishes neonatal septic arthritis from Perthes disease on ultrasound is:
- A Staphylococcus aureus; joint effusion with synovial thickening on ultrasound vs. avascular fragmentation in Perthes ✓
- B Group B Streptococcus; metaphyseal irregularity on X-ray
- C E. coli; lack of effusion on ultrasound
- D Candida; bilateral joint involvement
Explanation
Staphylococcus aureus is the most common causative organism of septic arthritis across all age groups including neonates, though neonates (especially premature) also have Group B Streptococcus and Gram-negative organisms (E. coli, Klebsiella) as frequent pathogens. Ultrasound is the investigation of choice in the neonate: it demonstrates hip joint effusion with synovial thickening and increased echogenicity of the joint fluid, which mandates emergency aspiration and surgical irrigation — the definitive diagnostic and therapeutic procedure. Perthes disease (avascular necrosis of the femoral head) is rare under age 4 and shows avascular fragmentation on radiographs (Catterall stages) but no effusion or systemic signs of infection.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.