The crystal at the end of the arrow is uric acid. When uric acid is thin and hexagonal, it can resemble cystine. To make sure the crystals are really cystine and not uric acid, add sodium cyanide to the sample, which reduces cystine to cysteine. Free sulfhydryl groups react with nitroprusside to form a purple color. This is called the sodium-nitroprusside reaction. If the sample just has uric acid, then the purple color will not be present. In this photomicrograph, there are multiple forms of uric acid, none of which have the hexagonal shape of cystine.