Final answer:
The incorrect pair is the statement linking 'broad spectrum' with 'an organism susceptible to a wide range of medications.' The right concept for broad spectrum is an antimicrobial that targets a wide variety of pathogens.
Step-by-step explanation:
The pair that is INCORRECT is: 4) Broad spectrum - an organism that is susceptible to a wide range of medications. The correct association should be that a broad spectrum describes an antimicrobial that targets a wide variety of bacterial pathogens. These medications are used to treat infections when the specific pathogen has not been identified or when mixed infections are present.
Other pairs are correct: 1) Therapeutic index is indeed a measure of the relative toxicity of a medication. 2) A short half-life implies that a medication needs to be taken relatively frequently to maintain its therapeutic effect. 3) A 70S ribosome is a valid target of some antibacterial medications because it is part of the prokaryotic bacterial cell machinery, and therefore, not found in human cells. 5) Bacteriostatic implies that a medication inhibits the growth of bacteria, rather than killing them outright, as would a bactericidal agent.