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CPPD arthritis — SCE Rheumatology MCQ

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HardCrystal arthropathiesCPPD arthritisSCE Rheumatology

A 78-year-old woman develops acute painful knee swelling after admission with pneumonia. Examination shows chondrocalcinosis is seen on knee radiograph. Investigations show synovial fluid shows weakly positively birefringent rhomboid crystals. What is the most likely diagnosis?

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Correct answer: D — Acute CPP arthritis caused by calcium pyrophosphate crystals

The best answer is “Acute CPP arthritis caused by calcium pyrophosphate crystals”. The crystal morphology and clinical context identify Acute CPP crystal arthritis as the diagnosis. Septic arthritis is an important mimic in acute monoarthritis, but the crystal findings and broader context make the answer clear in this vignette. Finding crystals does not completely exclude infection when the patient is systemically unwell. The competing options represent related diagnoses, tests or treatments, but they do not match the decisive phenotype, safety constraint or management point in this stem.

Reference: EULAR CPPD diagnosis recommendations: https://ard.bmj.com/content/70/4/563