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Septic Arthritis — RACP Adult Medicine MCQ

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ModerateInfectious DiseasesSeptic ArthritisRACP Adult Medicine

A 40-year-old man with diabetes presents with an acutely hot swollen knee. He has a skin wound near the knee. Joint aspirate: WCC 85000 cells/μL (95% PMNs), no crystals, Gram stain shows gram-positive cocci in clusters. Serum WCC 18 × 10⁹/L, CRP 220 mg/L. What is the most likely diagnosis?

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Correct answer: C — Septic arthritis

Very high synovial WCC (>50000, predominantly PMNs), positive Gram stain (S. aureus), elevated systemic inflammatory markers, and a potential skin entry point is septic arthritis. This is an orthopaedic emergency requiring urgent arthroscopic washout and IV antibiotics (flucloxacillin for S. aureus). Any delay increases risk of joint destruction. Blood cultures and synovial fluid culture guide antibiotic therapy.

Reference: eTG Antibiotic – 2025 – Septic Arthritis; eTG Rheumatology 2024