Final answer:
The term that describes the number of animals responding to a drug dose with a significant effect is the Median Effective Dose (ED50), which is different from the lethal dose-50% (LD50). The correct answer is option b) Median Effective Dose (ED50).
Step-by-step explanation:
The term describing the number of animals that respond to a dose of drug is known as Median Effective Dose (ED50). The ED50 is defined as the dose that results in a significant response in 50% of the subjects. This is not to be confused with the lethal dose-50% (LD50), which is the dose that kills half (50%) of the animals tested.
The LD50 value is a common measure to assess the acute toxicity of chemicals, whereas the ED50 is concerned with the point at which 50% of the population exhibits a significant therapeutic response.
The correct answer to the student's question would be option b) Median Effective Dose (ED50), aligning with the definition provided in multiple sources, linking the ED50 to a dose that causes a significant response (neither lethal nor toxic) in 50% of the test animals. Therefore, the answer to the question "5. 'Effective dose - 50%' describes which one of the following?" is D. The dose that results in a significant response in 50% of subjects.