<prompt> A 63-year-old man develops pain and swelling in his knee. It appears warm, red, and swollen with decreased range of movement. A diagnostic aspiration is performed. Which of the following will most likely distinguish pseudogout 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. 