A 60-year-old woman presents with acute onset of painful swelling in the right knee. Arthrocentesis yields cloudy synovial fluid with a leukocyte count of 58,000/mm3 (92% neutrophils). Weakly positively birefringent rhomboid-shaped crystals are seen under polarized light microscopy. Gram stain is negative. Which of the following is the most likely crystal identified?

• A Monosodium urate crystals
• B Calcium oxalate crystals
• C Calcium pyrophosphate dihydrate crystals ✓
• D Hydroxyapatite crystals

Explanation

Calcium pyrophosphate dihydrate (CPPD) crystals are rhomboid-shaped and show weak positive birefringence under compensated polarized light. This distinguishes them from monosodium urate crystals, which are needle-shaped and show strong negative birefringence. CPPD deposition disease (pseudogout) commonly affects the knee in older adults. Synovial fluid WBC counts can mimic septic arthritis (often >50,000), making crystal identification 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.