Final answer:
Rhomboid-shaped, positively birefringent crystals on joint fluid aspirate are characteristic of pseudogout, where they appear rectangular under a microscope, helping to establish the diagnosis.
Step-by-step explanation:
The characteristic feature of rhomboid-shaped, positively birefringent crystals on joint fluid aspirate is that they are typically seen in cases of pseudogout, where calcium pyrophosphate dihydrate (CPPD) crystals are present. These crystals appear as rhomboid-shaped and are strongly birefringent, meaning they will appear differently when viewed under polarized light in comparison to negatively birefringent crystals such as those seen in gout, which are needle-shaped and not rhomboid. In the given options, the correct characteristic feature of these crystals would be that they are rectangular, as rhomboid crystals can exhibit a rectangular appearance under the microscope. It is important to correctly identify these crystals in the clinical setting as they help in establishing the diagnosis of pseudogout.