What type of crystals are found in joints affected by gout?

• A. Monosodium urate crystals
• B. Calcium pyrophosphate crystals
• C. Hydroxyapatite crystals
• D. Cholesterol crystals

Answer: (A)

Gout is driven by deposition of monosodium urate crystals in joints when uric acid levels are high. In synovial fluid, these crystals are needle-shaped and show negative birefringence under polarized light, meaning they appear yellow when aligned parallel to the slow axis and blue when perpendicular. This crystal morphology and birefringence pattern are characteristic and distinguish gout from other crystal arthropathies. Calcium pyrophosphate dihydrate crystals, by contrast, are rhomboid and positively birefringent, which is typical of pseudogout. Hydroxyapatite crystals are associated with calcific tendinopathy rather than the classic joint crystals of gout. Cholesterol crystals are not involved in gouty joints.