A 68-year-old man has an acute monoarthritis of the right knee with turbid synovial fluid. Polarised light microscopy shows positively birefringent rhomboid crystals. Which of the following statements about this condition is CORRECT?

• A Crystals are monosodium urate; treated with colchicine alone
• B Crystals are calcium oxalate; associated with renal failure
• C Crystals are calcium pyrophosphate; the condition is called pseudogout and may indicate hyperparathyroidism ✓
• D Crystals are hydroxyapatite; treatment is corticosteroid injection only

Explanation

Positively birefringent rhomboid crystals under compensated polarised light microscopy are calcium pyrophosphate dihydrate (CPPD), causing pseudogout. In contrast, gout crystals (monosodium urate) are negatively birefringent and needle-shaped. Chondrocalcinosis/CPPD deposition can be secondary to metabolic conditions: hyperparathyroidism, haemochromatosis, hypomagnesaemia, hypophosphataemia, Wilson's disease. Investigation for underlying metabolic disease (serum calcium, PTH, ferritin, magnesium, phosphate) is warranted in patients under 55 or with polyarticular disease.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.