A 65-year-old male develops the sudden onset of severe knee pain. The knee is red, swollen, and tender. He has a history of diabetes mellitus and cardiomyopathy. An x-ray of the knee shows linear calcification. Definitive diagnosis is best made by
Serum uric acid Serum calcium Ahrocentesis and identification of positively birefringent rhomboid crystals Rheumatoid factor