Which statement about toxicity is true?

• A. The dose makes the poison
• B. All substances are toxic at any dose
• C. Toxicity depends only on the exposure route
• D. Toxicity is only a function of airborne concentration

Answer: (A)

The main idea being tested is that toxicity depends on the dose—how much of a substance actually enters the body and interacts with it. This is the principle that “the dose makes the poison.” A substance can be completely harmless at a tiny amount but become toxic as the dose increases, because the body’s defense systems can be overwhelmed or the substance can reach its harmful target in larger quantities. The concept also acknowledges that there isn’t a single universal toxic dose for all substances; each chemical has its own dose–response relationship and individual factors like metabolism and age matter.

For example, water is essential for life, yet drinking an extreme amount in a short time can be dangerous because it disrupts electrolyte balance. Conversely, a tiny amount of a highly toxic chemical can be lethal if absorbed efficiently or if someone is especially sensitive. This illustrates why the statement asserting that the dose determines toxicity is the best answer.

The other ideas don’t fit because they imply toxicity is governed by a single factor. Route of exposure matters, but it isn’t the sole determinant—how much actually gets into the body and for how long, plus how the body processes it, all shape toxicity. And focusing only on an airborne concentration ignores the total dose and the body’s response, which come from multiple exposure routes and durations.