## Most Common Causative Agent of Acute Hematogenous Osteomyelitis **Key Point:** Staphylococcus aureus is the single most common organism causing acute hematogenous osteomyelitis in children across all age groups, accounting for 50–90% of cases. ### Why S. aureus Dominates **High-Yield:** S. aureus has a predilection for the metaphyseal region of long bones due to: - Slow blood flow in metaphyseal capillaries - Ability to produce bone-degrading enzymes (coagulase, hyaluronidase) - Tropism for bone tissue via fibronectin-binding proteins ### Organism Characteristics | Feature | S. aureus | S. pneumoniae | E. coli | H. influenzae | | --- | --- | --- | --- | --- | | **Frequency in children** | 50–90% | <5% | <5% (neonates) | Rare (post-vaccine) | | **Age group** | All ages | Rare in osteomyelitis | Neonates, immunocompromised | Rare now | | **Route** | Hematogenous, direct inoculation | Hematogenous (rare) | Ascending UTI (neonates) | Hematogenous (rare) | | **Bone site** | Metaphysis of long bones | Variable | Metaphysis | Variable | ### Clinical Pearl **Mnemonic:** **STAPH** = **S**taphylococcus **A**ureus is **P**rimary **H**ematogenous pathogen In the era of Hib vaccination, S. aureus remains unchallenged as the leading cause. Methicillin-resistant S. aureus (MRSA) is increasingly common in community-acquired osteomyelitis. ### Age-Based Variations - **Neonates (<3 months):** S. aureus, Group B Streptococcus, E. coli - **Infants & children (3 months–5 years):** S. aureus (>80%) - **Older children & adolescents:** S. aureus (>80%) **Warning:** Do not confuse with septic arthritis in neonates, where Group B Streptococcus is more common. [cite:Rockwood & Green's Fractures in Adults 9e Ch 1] 
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