The lowest concentration of drug that will kill all but the minimum defined proportion of viable organisms after incubation for a fixed time under a given set of conditions is the definition of the:

The description provided in the question refers to the concept of determining a critical threshold of drug concentration, which effectively demonstrates the ability of a drug to eradicate bacteria. The term that accurately encompasses the lowest concentration of a drug that effectively kills all but a specific proportion of viable organisms is known as the minimum bactericidal concentration (MBC).

The minimum lethal concentration is often used interchangeably with MBC, as they both represent the lowest drug concentration that results in bactericidal effects. However, the term "minimum bactericidal concentration" specifically highlights the relationship between drug concentration and the bacterial viability, making it the more precise term in the context of bacteriology. Recognizing that both terms may describe similar concepts is why the correct answer includes both terms together in this case.

Thus, the answer incorporates the idea that this concentration is crucial for understanding the efficacy of antimicrobial agents. Understanding these definitions is paramount in the study of drug efficacy against bacterial infections, which supports the choice of including both the minimum lethal concentration and minimum bactericidal concentration as correct interpretations of the description provided in the question.