A patient with gout presents with an acutely inflamed joint. What would aspiration of the synovial fluid reveal?

Correct answer: B. Needle-shaped monosodium urate crystals showing negative birefringence

• A. Cholesterol crystals appear in chronic effusions such as long-standing rheumatoid arthritis, not in acute gout.
• B. CORRECT. Gout is caused by deposition of MONOSODIUM URATE crystals in the joint. Under POLARISED LIGHT MICROSCOPY they are NEEDLE-SHAPED and NEGATIVELY birefringent, appearing YELLOW when aligned parallel to the axis of the compensator. Joint aspiration demonstrating these crystals is the diagnostic gold standard — serum urate may be normal during an acute attack and is not diagnostic on its own.
• C. RHOMBOID, POSITIVELY birefringent crystals are CALCIUM PYROPHOSPHATE — that is PSEUDOGOUT (CPPD), the principal differential diagnosis and a favourite distractor.
• D. Creatinine is a serum marker of renal function. It is not what is sought in synovial fluid, although chronic hyperuricaemia and renal impairment frequently coexist.

Note: Memory hook — the two crystal arthropathies: gout = Needle, Negative birefringence, moNosodium urate; pseudogout = rhomboid, Positive, calcium Pyrophosphate. Dental relevance: gout patients are often on colchicine, allopurinol or febuxostat, and ASPIRIN at low dose reduces urate excretion and can precipitate an attack — choose a different analgesic.