In which situation would a patient show negative birefringence in the joint fluid?

In the context of this question, negative birefringence in joint fluid is indicative of the presence of monosodium urate crystals, which are commonly associated with gout. When viewed under polarized light microscopy, these crystals display negative birefringence, meaning they appear yellow when aligned parallel to the compensator and blue when perpendicular to it. This optical property is key in diagnosing gout, as it distinguishes it from other types of crystal-induced arthropathies.

Pseudogout, on the other hand, is linked to the presence of calcium pyrophosphate dihydrate (CPPD) crystals, which exhibit positive birefringence, appearing blue when parallel and yellow when perpendicular. Conditions like osteoarthritis and rheumatoid arthritis do not typically involve the presence of crystals in the synovial fluid that would demonstrate birefringence characteristics; instead, they are characterized by other types of inflammatory or degenerative changes. Thus, the confirmation of negative birefringence in joint fluid is a strong signal for diagnosing gout specifically.