Synovial fluid analysis in an elderly woman with acute knee monoarthritis reveals rhomboid-shaped, weakly positively birefringent crystals under polarised light microscopy. Which crystal is this?

• A Monosodium urate
• B Basic calcium phosphate (hydroxyapatite)
• C Calcium oxalate
• D Calcium pyrophosphate dihydrate ✓

Explanation

Calcium pyrophosphate dihydrate (CPPD) crystals are rhomboid or rod-shaped and show weak positive birefringence, the mirror image of gout, where monosodium urate crystals are needle-shaped with strong negative birefringence. Calcium oxalate is seen in renal failure patients on dialysis, while basic calcium phosphate crystals are non-birefringent and require special stains, so neither matches the described morphology.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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