A 68-year-old man with a history of hypertension and atrial fibrillation presents with acute monoarthritis of the right knee. Joint aspiration yields cloudy fluid with 28,000 WBC/µL. Under polarized light microscopy, the fluid shows weakly positively birefringent rod-shaped crystals. Which is the most likely underlying diagnosis?
- A Gout
- B Basic calcium phosphate (hydroxyapatite) crystal disease
- C Calcium pyrophosphate deposition disease (CPPD) ✓
- D Infectious (septic) arthritis
Correct answer: C. Calcium pyrophosphate deposition disease (CPPD)
Explanation
Positively birefringent rod-shaped or rhomboid crystals under compensated polarized light are characteristic of calcium pyrophosphate deposition disease (CPPD, pseudogout). Gout crystals are negatively birefringent and needle-shaped. Hydroxyapatite crystals are non-birefringent and require alizarin red staining. Septic arthritis would show no crystals and Gram stain or culture would be positive.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.