A 76-year-old woman develops an acutely swollen, painful right knee two days after hip replacement surgery. Synovial fluid shows 18,000 cells/mm3 with predominantly neutrophils. Compensated polarised light microscopy reveals rhomboid-shaped crystals showing weak positive birefringence. Radiograph of the knee shows linear calcification of the menisci. What is the causative crystal?
- A Monosodium urate
- B Basic calcium phosphate (apatite)
- C Calcium pyrophosphate dihydrate ✓
- D Calcium oxalate
Explanation
Rhomboid or rod-shaped crystals with weak positive birefringence on compensated polarised microscopy are diagnostic of calcium pyrophosphate dihydrate deposition, causing acute pseudogout. Meniscal chondrocalcinosis on radiograph supports the diagnosis, and flares are commonly triggered by surgery or serious illness in elderly patients. Monosodium urate crystals are needle shaped with strong negative birefringence, which distinguishes gout. Basic calcium phosphate gives an amorphous deposit seen only by electron microscopy, and calcium oxalate is bipyramidal and seen mainly in dialysis patients.
Reference: Kelley's Textbook of Rheumatology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.