A 69-year-old man has had mild arthritis involving many joints for several years. Over 1 or 2 days, he develops severe pain and swelling of his knee. His x-ray reveals calcifications in his articular cartilage. Which of the following is the most likely diagnosis?

The articular calcification chondrocalcinosis is typical for pseudogout or calcium pyrophosphate disease (CPPD). The most common joint involved is the knee, but the wrist, shoulder, ankle, elbow, and hand are also frequently involved. Definitive diagnosis depends on finding typical rhomboid-shaped crystals with weak-positive birefringence in the synovial fluid, but chondrocalcinosis in the correct setting allows a presumptive diagnosis. Numerous diseases are associated with CPPD, but the most common predisposing factor is advancing age.