Questions: Minimum inhibitory concentration is Highest concentration of a drug with the lowest toxic dose to the host Smallest concentration of a drug with the highest toxic dose to the host Highest dilution of a drug that inhibits microbial growth Lowest dilution of a drug that inhibits microbial growth

The answer is the fourth one: Lowest dilution of a drug that inhibits microbial growth.

Explanation for each option:

1. Highest concentration of a drug with the lowest toxic dose to the host - This is incorrect because the minimum inhibitory concentration (MIC) is not related to the toxic dose to the host but rather to the effectiveness of the drug against microbes.
2. Smallest concentration of a drug with the highest toxic dose to the host - This is incorrect for the same reason as the first option; MIC is not about the toxic dose to the host.
3. Highest dilution of a drug that inhibits microbial growth - This is incorrect because the MIC refers to the lowest concentration, not the highest dilution.
4. Lowest dilution of a drug that inhibits microbial growth - This is correct because the MIC is defined as the lowest concentration of an antimicrobial agent that will inhibit the visible growth of a microorganism after overnight incubation.

Summary: The minimum inhibitory concentration (MIC) is the lowest dilution of a drug that inhibits microbial growth.