A 72-year-old woman presents to the emergency department with a 2-day history of severe pain, redness, and swelling in her left knee. She has no history of trauma. On examination, the left knee is warm and tender with an effusion. Plain radiographs of the knee demonstrate calcification of the meniscal cartilage (chondrocalcinosis). Synovial fluid aspiration yields a turbid fluid, and polarized light microscopy reveals rhomboid-shaped crystals that are weakly positively birefringent. Which of the following is the most likely composition of these crystals?