If you are performing this test on an unknown organism why is it a good idea to run simultaneous

The question seem to be incomplete but I found the complete question which is:
If you are performing this test on an unknown organism, why is it a good idea to run simultaneous tests on known phenylalanine-positive and phenylalanine-negative organisms?

Here is the

Inoculation of a positive control and success from it includes certainty to negative outcomes on an unknown organism. That is, you know the test is working effectively, so the negative outcome is most likely precise. Without the positive control, there is dependably a component of uncertainty whether the negative outcome is a genuine negative or a false negative. performing the test on a known phenlalanine-negative organism is valuable in that it exhibits what a negative outcome looks like.

Where are they though?