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Acute CPP crystal arthritis — SCE Rheumatology MCQ

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ModerateCrystal arthropathiesAcute CPP crystal arthritisSCE Rheumatology

A 52-year-old woman has acute painful knee swelling after an intercurrent illness. Examination shows there is a warm effusion without trauma. Investigations show synovial fluid shows rhomboid weakly positively birefringent crystals. The consultant wants the next test most likely to confirm the suspected process or guide treatment. The result will change diagnosis, staging or immediate treatment. What is the most appropriate investigation?

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Correct answer: B — Calcium, phosphate and magnesium

Calcium, phosphate and magnesium is the best answer because calcium, phosphate and magnesium is the investigation most likely to resolve the immediate diagnostic or monitoring question in acute cpp crystal arthritis. Ultrasound-guided aspiration is a plausible distractor, but it does not account for the key discriminator in the stem. Renal function and DXA scan would be more appropriate in a different clinical pattern or at another point in the pathway, while Autoantibody profile is suboptimal for this presentation. Clinical pearl: autoantibodies are supportive tests and should be interpreted alongside phenotype, inflammatory markers, urinalysis and imaging.

Reference: NICE NG219; BSR hot swollen joint guideline; EULAR crystal arthropathy recommendations