Testing for Cystine stones?!

[Silverish]

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First Aid says you do a Sodium cyanide nitroprusside test (+).
Does anyone have a clue why? I have no idea why.

 

[plasmodium]

Cyanide converts cystine to cysteine and, when present, nitroprusside binds cysteine. The binding of cysteine by nitroprusside elicits a deep purple color, which is interpreted as cystine being present, and thus a positive test result.