Which of the following is a symptom of exposure to highly toxic substances?

Exposure to highly toxic substances can lead to various adverse health effects, and one of the primary symptoms experienced is nausea and vomiting. These symptoms are part of the body's defense mechanism, often indicating that it is reacting negatively to a harmful substance. The gastrointestinal system can be particularly sensitive to toxins, and nausea serves as a warning sign that something is wrong, potentially prompting an individual to expel the ingested toxin. In many cases, nausea and vomiting can occur shortly after exposure, reflecting the acute reaction of the body to harmful chemicals or poisons.

In contrast, the other options do not align with typical responses to toxic exposure. Improved cognitive function and enhanced respiratory function are not expected outcomes when dealing with high levels of toxins; instead, exposure tends to impair or debilitate these functions. Similarly, increased appetite is not a recognized symptom following exposure to toxic substances; rather, it can lead to a loss of appetite as the body prioritizes dealing with the toxicity. Thus, nausea and vomiting are key indicators of the body's negative response to toxic exposure, making this answer the most accurate in the context of the question.