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Postoperative Monoarthritis — SCE Rheumatology MCQ

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ModerateRheumatology EmergenciesPostoperative MonoarthritisSCE Rheumatology

A 65-year-old man with rheumatoid arthritis treated with etanercept develops an acutely painful, hot and swollen ankle 4 days after cardiac surgery. His temperature is 38.5°C. Synovial-fluid microscopy demonstrates negatively birefringent monosodium urate crystals, but no organisms are seen on Gram staining; cultures are pending. Which diagnosis must still be excluded urgently?

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Correct answer: A — Septic arthritis

Septic arthritis must still be excluded urgently. Fever, recent surgery and biological immunosuppression increase concern for infection. Monosodium urate crystals establish that gout is present but do not establish that it is the only process: septic and crystal arthritis can coexist. A negative Gram stain is insufficiently sensitive to exclude infection, so synovial and blood cultures remain essential and the patient should follow the urgent local septic-arthritis pathway. Perioperative stress makes gout plausible, which is why option E is tempting, but it cannot safely account for the presentation until infection is excluded. Postoperative haemarthrosis may cause swelling but is less consistent with pyrexia. Reactive arthritis and a monoarticular RA flare are possible causes of inflammation but should not take priority over excluding destructive native-joint infection.

Reference: Smith C et al. Multi-centre evaluation of Gram stain in the diagnosis of septic arthritis. 2025. https://pubmed.ncbi.nlm.nih.gov/40330095/