A 68-year-old man presents with acute onset of severe pain, warmth, and swelling of the right knee. Joint aspiration shows weakly positive birefringent crystals under polarised light microscopy. X-ray of the knee shows chondrocalcinosis. What is the most likely diagnosis and causative crystal?
- A Pseudogout; calcium pyrophosphate dihydrate crystals ✓
- B Gout; monosodium urate crystals
- C Hydroxyapatite deposition disease; hydroxyapatite crystals
- D Septic arthritis; no crystals expected
Correct answer: A. Pseudogout; calcium pyrophosphate dihydrate crystals
Explanation
Pseudogout (calcium pyrophosphate crystal deposition disease, CPPD) presents with acute mono- or oligoarthritis, most commonly the knee in older adults, and is characterised by weakly POSITIVE birefringent crystals (appear blue when parallel to the compensator) under polarised light microscopy. Chondrocalcinosis on X-ray is the radiographic hallmark. Monosodium urate crystals in gout are NEGATIVELY birefringent (yellow when parallel to axis).
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.