<prompt> A 63-year-old man presents to the emergency room with severe pain and swelling in his right knee. The pain started suddenly, and there is no history of injury to the knee. On physical examination, the knee is warm, red, and swollen with decreased range of movement. A diagnostic aspiration of the knee is performed. Which of the following will most likely distinguish pseudo-gout from gout?positive birefringent crystalsacute onsetinvolvement of single jointsinvolvement of large joints  <response>: Pseudogout calcium pyrophosphate crystals - CPPD is distinguishable from gout by positive birefringent crystals. CPPD are short, blunt rhomboids, and urate crystals seen in gout are needle-shaped with negative birefringence. 