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Septic Arthritis — RACP Adult Medicine MCQ

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EasyRheumatologySeptic ArthritisRACP Adult Medicine

A 45-year-old man presents with acute monoarthritis of the knee. He is febrile (38.8°C). Joint aspirate is turbid with WCC 85 × 10⁹/L (>90% neutrophils). Gram stain shows gram-positive cocci in clusters. Crystals are absent. What is the most likely diagnosis?

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Correct answer: C — Septic arthritis

Acute monoarthritis with fever, turbid synovial fluid with very high WCC (>50 × 10⁹/L, >90% neutrophils), and organisms on Gram stain confirms septic arthritis. S. aureus is the commonest organism. This is a medical emergency requiring IV antibiotics (flucloxacillin if MSSA) and urgent joint washout (arthroscopic or open).

Reference: eTG Antibiotic – 2025 – Septic Arthritis