What should a positive cyanide-nitroprusside test indicate?

A positive cyanide-nitroprusside test is indicative of the presence of cystine in the urine. This test works by detecting cystine through the formation of a red-purple color when cystine reacts with sodium nitroprusside in the presence of cyanide. Cystine is an amino acid, and its presence may suggest conditions such as cystinuria, a genetic disorder affecting amino acid transport.

The test is specific for cystine, distinguishing it from other types of crystals, which do not elicit a positive reaction. Therefore, among the options provided, the correct identification aligns with the characteristics of cystine crystals, making it the most appropriate answer. The other substances listed—struvite, tyrosine, and cholesterol crystals—are not detected by this particular reaction, highlighting the specificity of the cyanide-nitroprusside test in identifying cystine in urine.