LD50 best describes which of the following?

• A. The dose required to produce lethality in 50% of the subjects.
• B. The dose that causes death in all animals.
• C. The dose that yields no lethal effect.
• D. The dose at which 75% survive.

Answer: (A)

The main concept is the median lethal dose. LD50 is the amount of a substance that will kill about half of the test subjects, determined from a dose–response relationship where mortality rises as dose increases. This point on the curve represents the dose at which 50% die, making it a standard measure of acute toxicity and a basis for comparing how toxic different substances are under the same conditions. The other descriptions describe different ideas: a dose that causes death in all animals would be an LD100, a dose with no lethal effect isn’t the LD50, and a dose where 75% survive corresponds to a different percentile of mortality, not the 50% mark.