Which of the following is considered a life-threatening sign of poison exposure?

Trouble breathing is considered a life-threatening sign of poison exposure because it indicates that the body is not receiving enough oxygen, which can lead to severe consequences, including brain damage or death, if not addressed immediately. This symptom can result from various poisons, especially those that affect the respiratory system or displace oxygen in the blood.

In contrast, symptoms like nausea, headache, and vomiting, while uncomfortable and potentially serious depending on the context, are not immediately life-threatening. They may indicate a range of less severe issues, and while they can be serious, they do not carry the same urgent risk to life as trouble breathing does. Recognizing breathing difficulties as a critical symptom allows for prompt action, which is essential in situations of poisoning.