Acute monoarthritis of the great toe in a patient with hepatitis C

Correct answer: C. Needle-shaped, negatively birefringent monosodium urate crystals

• A. Creatinine is a serum marker of renal function. It is not measured in synovial fluid and has no diagnostic role here.
• B. Cholesterol clefts appear in chronic effusions and in some cyst contents. They are not a feature of crystal arthropathy.
• C. CORRECT. Gout is deposition of MONOSODIUM URATE, the end product of PURINE catabolism, in and around joints. Under polarized light the crystals are NEEDLE-shaped and NEGATIVELY birefringent — yellow when aligned parallel to the compensator axis. Synovial fluid aspiration remains the definitive diagnostic test, because serum urate can be normal during an acute attack.
• D. Rhomboid, POSITIVELY birefringent crystals are CALCIUM PYROPHOSPHATE — pseudogout, which favours the knee and wrist rather than the first metatarsophalangeal joint and is associated with chondrocalcinosis on radiographs.

Note: What this case adds is the CRYSTAL identification and the birefringence pair. Mnemonic: gout is Needle-shaped and Negative; pseudogout is Positive and rhomboid. Dentally, avoid NSAIDs for an acute attack in this patient given the hepatitis and the alcohol history, and note that allopurinol has an interaction worth remembering with azathioprine.