Which of the following best defines a toxin?

• A. A disease-causing microorganism
• B. Poisonous substances produced by living organisms
• C. A synthetic chemical used in disinfectants
• D. A non-biological hazard

Answer: (B)

A toxin is a poisonous substance produced by a living organism. That origin is what sets toxins apart: they are the poisons made by biological sources, not the organism itself or a synthetic chemical. So, toxins are not disease-causing microorganisms (which are living organisms, not just poisons they produce), nor are they synthetic chemicals used in disinfectants (these are human-made and not inherently produced by living beings), nor are they non-biological hazards (which are not derived from living organisms). Examples like botulinum toxin from bacteria or ricin from a plant illustrate how a living source can generate a highly potent poison. The key idea is that toxins come from living organisms and act as poisons, often at very small doses.