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

Trouble breathing is identified as a life-threatening sign or symptom of poison exposure because it can indicate that the poison is affecting the respiratory system. This impairment can lead to inadequate oxygen delivery to vital organs, which can quickly result in serious complications or death. Immediate medical attention is required if a person exhibits this symptom after potential exposure to poison.

In contrast, while a skin rash, headache, or nausea may be concerning symptoms and warrant medical evaluation, they do not typically pose the same immediate life-threatening risk as difficulty breathing. These symptoms may indicate a less severe reaction or a response to the toxin that does not compromise the airway or respiratory function directly. Thus, recognizing trouble breathing as a critical warning sign is essential for determining the severity of the situation and the urgency of the response required.