Pseudogout is caused by calcium pyrophosphate crystal deposition. It clinically resembles gout to some degree but tends to affect large joints such as the knee rather than small peripheral joints. Joint x-ray films indicate some differences from classic gout but are frequently not a sufficient basis for diagnosis. Synovial fluid examination discloses calcium pyrophosphate crystals, either within neutrophil cytoplasm or extracellularly. These appear as short small rectangular structures or short rods, but sometimes color-compensated polarized light (imparting a positive birefringence, blue on red background) is necessary for reliable differentiation from uric acid.