An elderly patient with a swollen knee and prior effusion has synovial fluid demonstrating calcium pyrophosphate crystals. What is the most likely diagnosis?

• A. Septic arthritis
• B. Acute synovitis
• C. Hemarthrosis
• D. Pseudogout

Answer: (D)

The key idea is recognizing calcium pyrophosphate crystal deposition in the joint fluid as pseudogout. In older adults, a swollen knee with a history of prior effusions is a classic setting for CPPD disease, and finding calcium pyrophosphate crystals in the synovial fluid clinches the diagnosis. These crystals are typically rhomboid-shaped and show weak positive birefringence under polarized light, which differentiates them from gout crystals (needle-shaped and negatively birefringent).

This finding makes septic arthritis less likely in the absence of fever, systemic signs, or purulent fluid, and it fits better than a generic acute synovitis or hemarthrosis, which would not show calcium pyrophosphate crystals. Thus, the most likely diagnosis is pseudogout.