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?: (A) positive birefringent crystals, (B) acute onset, (C) involvement of single joints, (D) involvement of large joints
Answer is A. 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.