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Acute monoarthritis — SCE Rheumatology MCQ

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ModerateCrystal arthropathiesAcute monoarthritisSCE Rheumatology

A 66-year-old man presents with a 24-hour history of a severely painful, hot and swollen knee. He has previously diagnosed gout and takes long-term prednisolone. His temperature is 38.2°C and CRP is 164 mg/L. The knee has a clinically detectable effusion and no overlying cellulitis. Which is the single most important investigation to establish the cause of this acute monoarthritis?

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Correct answer: C — Synovial fluid Gram stain, culture and crystal microscopy

The correct answer is C. Fever, marked CRP elevation and glucocorticoid-associated immunosuppression make septic arthritis a critical possibility despite the history of gout. Urgent aspiration permits Gram staining and culture for infection and polarised microscopy for monosodium urate or calcium pyrophosphate crystals. Crystal arthritis and septic arthritis may coexist, so finding crystals does not remove the need for culture. Blood cultures should also be obtained when infection is suspected, but they do not directly determine the cause of the joint effusion and are not a substitute for aspiration. Serum urate can fall during an acute gout flare and cannot exclude infection. Plain radiography and dual-energy CT may support structural or crystal-related diagnoses but cannot reliably exclude septic arthritis.

Reference: NICE guideline NG219, Gout: diagnosis and management, recommendations 1.1.3–1.1.8, 2022. https://www.nice.org.uk/guidance/ng219/chapter/Recommendations; Oxford University Hospitals NHS Foundation Trust, Joint Aspirates, reviewed 17 February 2026. https://www.ouh.nhs.uk/microbiology/a-z/joint-aspirates/