A 50-year-old man presents with acute monarthritis of the knee with large effusion. Synovial fluid analysis shows positively birefringent rhomboid-shaped crystals under polarised light. The diagnosis is:

• A Gout; caused by monosodium urate crystals
• B Septic arthritis; white cell count >100,000/mm³ with positive Gram stain
• C Pseudogout (calcium pyrophosphate deposition disease); caused by calcium pyrophosphate dihydrate crystals ✓
• D Hydroxapatite deposition disease; apatite crystals seen on electron microscopy

Explanation

Positively birefringent (parallel polarised light makes the crystal appear blue when aligned with the slow axis) rhomboid-shaped crystals are calcium pyrophosphate dihydrate (CPPD) crystals characteristic of pseudogout/CPPD disease. Gout crystals are monosodium urate — needle-shaped and negatively birefringent (appear yellow when aligned with the slow axis). Septic arthritis diagnosis requires Gram stain/culture. Hydroxyapatite crystals are not visible by standard light microscopy.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.