Which of the following is a characteristic of endotoxins?

Endotoxins are indeed characterized by their heat-stability, which distinguishes them as components of the outer membrane of Gram-negative bacteria, primarily composed of lipopolysaccharides (LPS). This structural configuration allows them to withstand higher temperatures during processing or sterilization, unlike many proteins, which can denature and lose their biological function when exposed to heat.

The other characteristics listed do not apply to endotoxins. For example, endotoxins are not proteins; rather, they are complex molecules made of lipid and polysaccharide components. Additionally, they are not actively secreted by bacteria; instead, they are released when the bacteria die and the cell wall breaks down. This release often occurs upon bacterial lysis, in contrast to exotoxins, which are secreted actively by some bacteria. Furthermore, while endotoxins can provoke an immune response, it is typically weaker and less specific compared to that elicited by exotoxins, which are proteins and can stimulate a robust antibody response. Therefore, the hallmark of endotoxins' stability at high temperatures correctly represents their unique nature among bacterial toxins.