To ensure that the most appropriate drug is being used to treat a pathogen, which would need to be done first?

Obtaining sensitivity testing

Explanation:

Pathogens may be defined as the organisms that causes diseases in other animals. The organisms that can acts as pathogen are bacteria, virus, fungi and plasmodium.

The drug preparation against the pathogens requires the sensitivity testing. This is done on the cultured microbes to evaluate the pathogen and to understand how might the different drug is capable to control the population of the particular organisms.

Thus, the answer is obtaining sensitivity testing.