A 65-year-old woman presents with acute monoarthritis of the left knee — sudden, severely painful, with large effusion and 39°C fever. Joint aspiration yields turbid fluid; microscopy shows weakly positive birefringent rhomboid crystals under compensated polarised light. Cultures are negative. The most appropriate immediate treatment is:

• A Joint aspiration alone followed by allopurinol to prevent recurrence
• B IV antibiotics for presumed septic arthritis, as cultures may be falsely negative
• C Intravenous urate-lowering therapy (rasburicase) for acute attack management
• D Colchicine, NSAIDs, or intra-articular corticosteroid injection for pseudogout (calcium pyrophosphate crystal deposition) ✓

Explanation

Weakly positive (blue when parallel to compensator) rhomboid crystals under compensated polarized light are calcium pyrophosphate dihydrate (CPPD) crystals, confirming pseudogout (calcium pyrophosphate crystal deposition disease, CPPD). Treatment of acute pseudogout is identical to acute gout: colchicine, NSAIDs, or intra-articular/systemic corticosteroids (corticosteroids preferred in elderly with CKD/GI risk). Allopurinol treats gout (urate crystals — negatively birefringent needle-shaped); it has no role in pseudogout. Urate crystals are negatively birefringent (yellow when parallel) — not seen here. Cultures are negative confirming it is not septic arthritis.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.