In the dose-response curve, the point where it intersects the abscissa below which there is no detectable effect is called the:

• A. Response coordinate
• B. Linear gradient
• C. Average gradient
• D. Threshold dose

Answer: (D)

The main idea is the threshold dose: the smallest dose that produces a detectable effect on the curve. On a dose–response plot, the x-axis is dose, and the curve often sits at zero effect until a certain point. The place where the curve first rises from zero marks the threshold dose—the minimum dose needed to elicit a measurable response. Any dose lower than this produces no observable effect. The other terms don’t describe this starting point: a response coordinate refers to the outcome level on the y-axis, while linear or average gradient describe the slope or rate of change of the curve, not where the effect begins.