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
[{'content': 'What are these crystals that were aspirated from the bursa of an elbow of a patient with rheumatoid arthritis? \nSelect and explain. \nA. Cholesterol, B. Calcium apatite, C. Calcium oxalate, D. Calcium pyrophosphate dihydrate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Answer A. CholesterolThe rectangular, platelike crystals with strong birefringence (both positive and negative) and notched corners are characteristic of cholesterol crystals. Cholesterol crystals are occasionally seen in the bursal fluid of patients who have rheumatoid arthritis.', 'role': 'assistant'}]