LD50 measures the dose required to cause death in what proportion of a test population?

• A. 50%
• B. 25%
• C. 75%
• D. 100%

Answer: (A)

LD50 is the median lethal dose—the amount of a substance that, under defined experimental conditions, would kill 50% of the test population. This value comes from dose–response data: as the dose increases, the proportion of individuals that die increases, and the LD50 is the dose at which half of the subjects are expected to die. It’s a traditional way to compare how toxic different substances are and to guide risk assessments, but it’s important to remember that the exact LD50 depends on specifics like the species, age, sex, route of exposure, and experimental conditions. Other lethal-dose figures exist (like LD25 or LD75), which refer to different fractions of fatalities, not the 50% point.