Which statement correctly defines toxins?

• A. They are harmful substances produced by living organisms.
• B. They are harmless byproducts produced by living organisms.
• C. They are non-biological compounds found in water.
• D. They are enzymes that neutralize pathogens.

Answer: (B)

Toxins are harmful substances produced by living organisms. They can be proteins, peptides, or small molecules that, at sufficient doses, disrupt cellular functions, damage membranes, or interfere with normal physiology. This harm is why toxins are associated with bacteria (exotoxins and endotoxins), plants (poisonous compounds), and animals (venoms).

The other statements don’t fit because a harmless byproduct isn’t a toxin, non-biological compounds found in water aren’t defined as toxins, and enzymes that neutralize pathogens are defenders rather than toxins.