This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology and Rheumatology, Subject name: Medicine
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? 
Select and explain. 
A. positive birefringent crystals, B. acute onset, C. involvement of single joints, D. involvement of large joints
A: positive birefringent crystals
Answer: A: positive birefringent crystals 
Explanation: 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.