Needle-shaped crystals found in a fluid aspirated from a patient's toe, which appear yellow on the y-axis when polarized, are indicative of:

Needle-shaped crystals that appear yellow when under polarized light are specifically indicative of monosodium urate crystals, which are associated with gout. In gout, these crystals form due to high levels of uric acid in the blood, leading to crystal formation in the joints, particularly the big toe, which is a common site for gout attacks.

The characteristic appearance of these crystals—thin, needle-shaped, and yellow under polarized light—is a key diagnostic feature. In contrast, crystals associated with other conditions, such as calcium pyrophosphate dihydrate crystals seen in pseudogout, tend to have a different morphology and exhibit a positive birefringence where they appear blue when aligned appropriately.

Thus, the findings of needle-shaped crystals in the synovial fluid from a patient's toe strongly point to a diagnosis of gout, making it the correct choice.