Final answer:
A narrow-spectrum antimicrobial drug is most appropriate when the causative agent has been identified, targeting a limited range of bacteria and minimizing the risk of antibiotic resistance and superinfections.
Step-by-step explanation:
A narrow-spectrum antimicrobial drug is most appropriate when the causative agent of an infection has been identified. It specifically targets a limited range of bacteria, such as gram-positive and some gram-negative bacteria. By using a narrow-spectrum drug, the risk of promoting antibiotic resistance and causing superinfections is minimized, as it does not harm the normal microbiota as much as broad-spectrum drugs do.