A 70-year-old woman presents with acute monoarthritis of the right knee. Arthrocentesis reveals 22,000 leukocytes/mm³. Under compensated polarized microscopy, the synovial fluid shows weak positive birefringence of rod-shaped and rhomboid crystals. What is the most likely diagnosis?

• A Gout
• B Hydroxyapatite deposition disease
• C Septic arthritis
• D Calcium pyrophosphate deposition disease (CPPD) ✓

Explanation

Calcium pyrophosphate dihydrate crystals appear as rhomboid or rod-shaped crystals with weak positive birefringence under compensated polarized light. Gout crystals are needle-shaped with strong negative birefringence. CPPD (pseudogout) commonly affects the knee in elderly patients. Hydroxyapatite crystals are not birefringent and require special stains. The 22,000 leukocytes overlap with both crystal arthropathies and septic arthritis, so crystal identification is essential.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.