Methicillin-Resistant Staphylococcus aureus Iliac Osteomyelitis with Cortical Breach Mimicking Septic Arthritis of the Hip in a Child: A Diagnostic Pitfall.

Introduction: Pelvic osteomyelitis is an uncommon but clinically important condition in children, often presenting with symptoms that closely mimic septic arthritis of the hip. Overlapping features, such as fever, severe hip pain, limp, and inability to bear weight, frequently lead to diagnostic delays and, in some cases, to unnecessary hip exploration. Early recognition is essential, particularly with increasingly aggressive presentations associated with methicillin-resistant Staphylococcus aureus (MRSA). Case Report: An 11-year-old girl presented with fever, acute left hip pain, and refusal to bear weight, raising a strong suspicion of septic arthritis. Hip ultrasonography showed a peri-hip collection, and urgent hip arthrotomy was performed; however, the joint was found to be normal. Further exploration revealed a cortical breach in the iliac bone with pus tracking into the surrounding musculature. Post-operative magnetic resonance imaging (MRI) demonstrated osteomyelitis of the ilium and ischium with multiple intermuscular abscesses. Culture confirmed MRSA. Management was complicated by contrast-induced acute kidney injury and vancomycin-associated rash, necessitating modification of antimicrobial therapy. Surgical drainage and targeted antibiotics resulted in complete functional recovery at 12 months. Conclusion: Pelvic osteomyelitis can convincingly mimic septic arthritis and may only be recognized when intraoperative findings are discordant with clinical expectations. MRI is indispensable for identifying pelvic sources of infection, particularly after a negative hip arthrotomy. Awareness of atypical features such as cortical breach, aggressive MRSA infection, and potential treatment-related complications is critical for timely diagnosis and optimal outcomes.