Which type of synovial fluid crystal is short, rhomboid positively birefringent (pseudogout/CPPD)?

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

Answer: (C)

Crystal analysis of synovial fluid relies on both shape and birefringence under polarized light to distinguish different crystal arthropathies. The crystals that cause pseudogout are calcium pyrophosphate dihydrate. They are characteristically short and rhomboid in shape and they exhibit positive birefringence. This combination—the short rhomboid form with positive birefringence—fits pseudogout, setting it apart from gout crystals, which are needle-shaped and negatively birefringent (monosodium urate). Cholesterol crystals have a different appearance and optical behavior, so they don’t match the description. So, the described crystal pattern points to calcium pyrophosphate dihydrate in pseudogout.