What is a key characteristic of a substance classified as a poison?

A substance classified as a poison is characterized by high acute toxicity, which means that it can cause significant harm or even death with a relatively small amount of exposure. Acute toxicity refers to the immediate harmful effects that result from a single or short-term exposure to a substance, often affecting vital systems in the body. This characteristic is critical in distinguishing poisons from other substances, as it highlights the potential for rapid and severe health effects upon exposure.

While other options touch on various attributes of substances, they do not capture the essence of what defines a poison as effectively. For instance, a low boiling point might indicate the physical properties of a substance rather than its toxicity level, and a substance that is harmless in small quantities may actually be classified as safe or non-toxic rather than a poison. Substances used primarily for medicinal purposes can also be toxic in higher doses, but their classification as poisons does not stem from their intended use but rather from their potential harmful effects. Therefore, the defining characteristic of a poison lies in its high acute toxicity.