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Septic arthritis mimic in rheumatoid arthritis — SCE Acute Medicine MCQ

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ModerateAcute Rheumatological and Dermatological EmergenciesSeptic arthritis mimic in rheumatoid arthritisSCE Acute Medicine

A 62-year-old woman is assessed on the acute medical unit. She has rheumatoid arthritis on biologic therapy and a single hot swollen knee with fever. CRP is markedly raised. Initial assessment includes relevant observations, bedside tests and urgent blood results. What is the most appropriate investigation?

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Correct answer: A — Urgent joint aspiration for Gram stain and culture

Urgent joint aspiration for Gram stain and culture is the best answer because immunosuppression and monoarthritis with fever require aspiration to exclude septic arthritis. Coagulation profile including fibrinogen and D-dimer is plausible, but it is less appropriate for the physiology or timing described in this stem. The other distractors either fit a neighbouring presentation, delay definitive treatment, or miss the key immediate risk. Clinical pearl: Do not assume a flare when one joint is acutely hot and systemic features are present.

Reference: NICE CKS rheumatoid arthritis; BOA guidance