Analysis of synovial fluid from the temporomandibular joint of a patient with severe joint pain reveals the presence of needle-shaped crystals. What substance are these crystals composed of?

Gout is caused by the deposition of monosodium urate (uric acid) crystals in synovial joints and periarticular tissues. These crystals are needle-shaped (acicular) and negatively birefringent under polarized light microscopy. Gout can affect the temporomandibular joint, though this is uncommon. The diagnosis is confirmed by finding urate crystals in synovial fluid. Calcium pyrophosphate crystals cause pseudogout (CPPD disease), which causes rhomboid-shaped, positively birefringent crystals.