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Suspected native-knee septic arthritis with calcium pyrophosphate crystals — MRCEM SBA MCQ

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HardProcedures, imaging and diagnosticsSuspected native-knee septic arthritis with calcium pyrophosphate crystalsMRCEM SBA

A 68-year-old man with type 2 diabetes presents with 24 hours of atraumatic right-knee pain, swelling and rigors. His temperature is 38.4°C, pulse 106/min and blood pressure 128/76 mmHg. He cannot bear weight, and passive knee movement is severely painful. CRP is 186 mg/L. The native knee has been aspirated before antibiotics; sufficient fluid has been sent for culture. Urgent microscopy shows calcium pyrophosphate crystals and no organisms on Gram stain. Blood cultures have not yet been taken. What is the most appropriate next management plan?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Take blood cultures, start empirical intravenous antibiotics and arrange urgent orthopaedic assessment.

This is a *diagnostic and management* decision, not a choice between two mutually exclusive diagnoses. Calcium pyrophosphate crystals explain a possible inflammatory arthritis, but they do not exclude concurrent septic arthritis. Rigors, fever, inability to bear weight and markedly raised CRP sustain substantial concern for infection. A negative synovial Gram stain cannot resolve that concern: a UK–Ireland multicentre study found that Gram stain missed many microbiologically positive cases. ([pubmed.ncbi.nlm.nih.gov](https://pubmed.ncbi.nlm.nih.gov/40330095/?utm_source=openai)) The joint has already been sampled for culture. Take blood cultures promptly, then begin empirical intravenous antibiotics and obtain urgent orthopaedic assessment for possible further drainage and inpatient care; treatment should not wait for culture confirmation. UK NHS guidance describes hospital treatment with intravenous antibiotics and joint drainage for septic arthritis. ([whittington.nhs.uk](https://www.whittington.nhs.uk/document.ashx?id=6094)) **B** correctly preserves culture sampling and specialist review but delays treatment until the final result. **C** substitutes MRI for an already obtained diagnostic aspirate and delays antibiotics; MRI might help if a deeper or alternative focus were suspected. **D** repeats an adequate aspiration without a new indication; repeat sampling could be appropriate if the first specimen were insufficient. **E** gives undue weight to the crystals and postpones assessment despite the continuing infection concern; crystal-directed treatment alone would fit a presentation in which septic arthritis had been adequately discounted. ([pubmed.ncbi.nlm.nih.gov](https://pubmed.ncbi.nlm.nih.gov/40330095/?utm_source=openai))

Reference: Hot Red Swollen Joint (May 2013) — https://www.whittington.nhs.uk/document.ashx?id=6094 Multi-centre evaluation of Gram stain in the diagnosis of septic arthritis (2025) — https://pubmed.ncbi.nlm.nih.gov/40330095/ Septic arthritis (3 September 2026) — https://www.nhs.uk/conditions/septic-arthritis/