Which of the following should be assessed first in a patient who has ingested an unknown substance?

In cases of toxicologic emergencies, particularly when a patient has ingested an unknown substance, assessing airway patency is crucial. The airway is the most critical aspect of a patient's basic life support, as it directly affects their ability to breathe and receive oxygen. If the airway is compromised due to swelling, secretions, or loss of consciousness, it can lead to respiratory failure within moments.

Maintaining an open airway allows the EMT to ensure that the patient can breathe adequately, which is essential for preventing hypoxia and other life-threatening complications. Once the airway is assessed and secured if necessary, further evaluations of the patient's circulatory status (including pulse rate), skin condition, and level of consciousness can follow, but the immediate priority remains the airway. This is in line with the principles of emergency medical care where immediate threats to life are addressed first.