Which condition is diagnosed by the presence of positively birefringent rhomboid-shaped crystals in joint fluid?

The presence of positively birefringent rhomboid-shaped crystals in joint fluid is indicative of pseudogout, which is also known as calcium pyrophosphate dihydrate (CPPD) crystal deposition disease. These crystals form when there is an abnormal accumulation of calcium pyrophosphate in the joint spaces, leading to inflammation and pain similar to gout but with distinct crystal characteristics.

In the context of crystal-induced arthropathies, gout is associated with negatively birefringent needle-shaped urate crystals, which are qualitatively different from the rhomboid-shaped crystals seen in pseudogout. Rheumatoid arthritis involves immune-mediated joint inflammation and does not present with crystals in the synovial fluid, while osteoarthritis is primarily characterized by degenerative joint changes rather than crystal deposition. Therefore, the identification of these specific crystals in joint fluid is crucial for accurately diagnosing pseudogout.