Healthy human gastric juice

First, the plasma of healthy people contains plasma protein, so the identification with biuret reagent will lead to purple reaction and error;

B, healthy people's saliva contains salivary amylase, which belongs to protein, so the identification with biuret reagent can lead to purple reaction, B error;

C, healthy people's gastric juice contains pepsin, which belongs to protein, so biuret reagent can be used to identify purple reaction, C error;

D, healthy urine contains urea, inorganic salts, etc. , and does not contain protein, biuret reagent identification can't have purple reaction, D is correct.

Therefore, choose: d.