A 78-year-old woman presents with acute onset of a hot, swollen right knee. She has a history of osteoarthritis. X-ray of the knee shows chondrocalcinosis. Synovial fluid analysis reveals rhomboid-shaped crystals with weak positive birefringence under polarized light microscopy. What is the most likely diagnosis?
- A Gout
- B Calcium pyrophosphate deposition disease ✓
- C Septic arthritis
- D Rheumatoid arthritis flare
Correct answer: B. Calcium pyrophosphate deposition disease
Explanation
Calcium pyrophosphate deposition disease (CPPD, pseudogout) is characterized by rhomboid, weakly positively birefringent crystals. Gout crystals are needle-shaped and negatively birefringent. Chondrocalcinosis on X-ray supports CPPD. Septic arthritis would have very high cell count and positive culture. RA flare is polyarticular and not associated with these crystals.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.