What does LD50 mean?

• A. Dose that causes 50% mortality in the test population.
• B. Dose required to produce observable symptoms in all individuals.
• C. Dose after which no additional effects are observed.
• D. Total lethal dose for the entire population.

Answer: (A)

LD50 is the median lethal dose—the amount of a substance that will kill 50% of a test population under defined conditions. This is why the statement describing death in half of the subjects at a certain dose is the correct description: it reflects the point on the dose–response curve where mortality reaches 50%. The value is typically expressed as milligrams of substance per kilogram of body weight and depends on factors like species, sex, route of exposure, and testing conditions, so it varies rather than being a universal constant. It does not indicate observable symptoms in all individuals, nor the dose at which no additional effects occur (that would be NOAEL or a similar concept), nor the total lethal dose for the entire population (LD100).