A 68-year-old woman presents with acute onset of pain, swelling, and warmth in the right knee. Synovial fluid analysis reveals weakly positively birefringent rhomboid-shaped crystals under polarized light microscopy. Plain radiograph of the knee shows linear calcification of the articular cartilage. What is the most likely diagnosis?
- A Monosodium urate gout
- B Calcium pyrophosphate deposition disease (CPPD) ✓
- C Basic calcium phosphate (hydroxyapatite) disease
- D Septic arthritis
Correct answer: B. Calcium pyrophosphate deposition disease (CPPD)
Explanation
CPPD (pseudogout) is characterized by weakly positively birefringent rhomboid-shaped calcium pyrophosphate crystals under polarized light. Chondrocalcinosis (calcification of articular cartilage) on radiograph is the hallmark. Monosodium urate crystals are needle-shaped and strongly negatively birefringent. Hydroxyapatite crystals are non-birefringent and seen on Alizarin red stain.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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