What is the definition of a toxin?

A toxin is specifically defined as a poisonous substance that is produced naturally by living organisms, such as plants, animals, or microorganisms. This definition encompasses various types of toxic substances that can cause harm to other living beings. Toxins can lead to a range of adverse health effects, depending on their nature and the amount of exposure.

The other options, while related to chemicals and substances, do not accurately capture the essence of what a toxin is. A non-harmful substance would not fit the definition, as it suggests a lack of toxicity. Likewise, a toxic chemical used in pesticides refers to a specific application of a chemical that can be toxic, but does not represent the broader definition of toxins. Finally, a beneficial organic compound cannot be classified as a toxin, as it implies a positive biological function rather than a harmful one. Thus, the characterization of a toxin being a poisonous substance is accurate and aligns with its recognized definition in biology and chemistry.