Why is an extra tube required when testing three amino acids for decarboxylase?

When testing for decarboxylase activity of amino acids, an extra tube is important for establishing a negative control. A negative control is necessary in experiments to ensure that any observed changes or outcomes can be attributed specifically to the reaction being tested, rather than to external contaminants or variables.

In this case, the extra tube typically contains a base medium without the amino acids being tested. This allows for the verification that any color change or other indicators present in the experimental tubes are indeed due to the decarboxylation of the amino acids and not a result of other factors. Thus, using a negative control provides a reliable benchmark for interpreting the results of the decarboxylase tests.

While the other options might suggest valid testing scenarios, such as assessing variations or confirming results, they do not specifically address the role of an extra tube in controlling for contamination or unexpected reactions that may confuse the interpretation of the results. The fundamental utility of a negative control is crucial in experimental design and helps ensure that the conclusions drawn from the data are valid and reliable.