A 72-year-old woman presents with acute monoarthritis of the knee. Joint fluid analysis shows rhomboid, rod-shaped crystals that are weakly positively birefringent under polarized light. Serum uric acid is 5.2 mg/dL. The most likely diagnosis is:
- A Acute gout due to monosodium urate deposition
- B Pseudogout due to calcium pyrophosphate dihydrate deposition ✓
- C Septic arthritis with calcium oxalate crystals
- D Basic calcium phosphate crystal arthropathy
Correct answer: B. Pseudogout due to calcium pyrophosphate dihydrate deposition
Explanation
Rhomboid or rod-shaped crystals showing weak positive birefringence are diagnostic of calcium pyrophosphate dihydrate (CPPD) deposition disease, or pseudogout. Monosodium urate crystals of gout are needle-shaped and strongly negatively birefringent. Calcium oxalate crystals appear as bipyramidal envelopes and occur mainly in dialysis patients, and basic calcium phosphate crystals are too small to be seen by light microscopy and require special staining. Her normal uric acid argues against gout.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.