On the basis of which factor an antimicrobial agent is selected?

A. Sensitivity of the microorganism to the drug: The primary goal is to use a "Culture and Sensitivity" test to determine which specific antibiotic effectively kills or inhibits the specific pathogen causing the infection. B. Bioavailability of the drug: This affects dosing and route, but it is not the primary factor in selecting which agent will be effective against the germ. C. Half-life of the medication: This determines how often the drug is dosed, not whether the drug is the correct choice for the infection. D. Therapeutic levels of the drug: This is a monitoring factor (like Vancomycin troughs) to ensure safety and efficacy after the drug has already been selected.