Septic Arthritis of the Hip in a Young Athlete: A Diagnostic Delay Caused by Cognitive Bias.

Septic arthritis requires urgent diagnosis and treatment; however, acute hip pain in young athletes may be misinterpreted as a sports-related musculoskeletal condition, resulting in delayed recognition. We report a 21-year-old male rugby player who presented with rapidly worsening left hip pain after a match and difficulty bearing weight. Laboratory tests showed elevated inflammatory markers. Plain radiographs were normal, while magnetic resonance imaging demonstrated a hip joint effusion. Fluoroscopy-guided aspiration yielded approximately 11 mL of cloudy yellow synovial fluid, but antibiotic therapy was not initiated because a sports-related injury was initially suspected. His symptoms and inflammatory markers temporarily improved with rest and anti-inflammatory medication, supporting the initial noninfectious impression. He was discharged but returned that evening with recurrent severe pain and an inability to walk. Repeat evaluation showed worsening inflammation, and contrast-enhanced computed tomography demonstrated persistent effusion with synovial enhancement. Synovial fluid culture subsequently grew methicillin-sensitive Staphylococcus aureus (MSSA). After MSSA was identified, targeted antibiotic therapy was started immediately, and arthroscopic irrigation and debridement were subsequently required. He successfully returned to competitive rugby 20 weeks postoperatively. This case highlights how anchoring bias, together with initial clinical ambiguity and premature closure, can delay the diagnosis of septic arthritis. In patients with acute hip pain, joint effusion and elevated inflammatory markers should prompt prioritization of infection in the differential diagnosis. Cloudy synovial fluid should trigger immediate diagnostic reassessment, and a diagnostic timeout may be valuable when the clinical course is inconsistent with the initial impression.