How is a bioassay used to determine the LD50?

• A. Administering different doses to a group of test organisms and observing the effects to determine the dose at which 50% of the subjects exhibit a specified adverse effect.
• B. Measuring the pH of a solution.
• C. Observing only the highest dose.
• D. Using a single dose and extrapolating.

Answer: (A)

The main concept is building a dose–response relationship to find the lethal dose for 50% of the population. In a bioassay for LD50, you expose several groups of test organisms to a range of doses, then observe whether each subject dies (within a defined time) or shows the specified adverse effect. By plotting the proportion affected against the dose and using appropriate statistics, you interpolate the dose that produces the effect in 50% of the subjects. This is why multiple dose levels are essential: they define how toxicity rises with dose and allow a precise LD50 estimate.

Why the other ideas don’t fit: measuring pH isn’t about toxicity or lethality; assessing only the highest dose misses the full dose–response trend; and using a single dose with extrapolation cannot reliably identify the 50% threshold and introduces a lot of uncertainty.