Final answer:
In crystal examination, monosodium urate shows needle-shaped negative birefringent crystals associated with gout, while calcium pyrophosphate dihydrate displays rhomboid or weakly positive birefringence commonly seen in pseudogout cases.
Step-by-step explanation:
On crystal examination, monosodium urate (MSU) shows needle-shaped negative birefringent crystals and calcium pyrophosphate dihydrate (CPPD) shows rhomboid or weakly positive birefringence. This aligns with the characteristics of crystals associated with gout, where MSU crystals are typically needle-shaped and exhibit negative birefringence, and pseudogout, where CPPD crystals are observed as rhomboid-shaped with varying birefringence properties.
Birefringent materials, such as the common mineral calcite, split unpolarized beams of light into two, resulting in an ordinary ray and an extraordinary ray. In the context of crystal examination for medical diagnosis, this behavior is utilized to differentiate between the types of crystals present in joint fluid samples. Notably, Figure 27.50 illustrates how birefringence in calcite does not comply with Snell's law, which is relevant when observing crystal formation in the medical examination.