On a dose-response curve, which description correctly identifies the LD50?

• A. The dose on the x-axis where 50% die (y-axis).
• B. The dose on the x-axis at which 50% respond on the y-axis.
• C. The dose corresponding to 50% lethality on the x-axis.
• D. The highest dose tested.

Answer: (C)

LD50 is the median lethal dose: the amount of substance that would kill half of a test population. On a dose–response curve, dose sits on the x-axis and percent mortality sits on the y-axis. To find the LD50, locate the 50% mortality level on the y-axis and read the corresponding dose from the x-axis. The description that states the dose corresponding to 50% lethality on the x-axis matches this reading precisely. It isn’t about the highest dose tested, and it isn’t just the point where 50% respond in general terms—the specific reading is the dose that yields 50% death.