Which of the following is a critical sign of cardiac arrest?

The correct answer is unresponsive with no pulse, as this is a definitive critical sign of cardiac arrest. When a patient is unresponsive, it indicates that there is a significant disruption in their neurological function, often caused by inadequate blood flow to the brain and vital organs due to the heart not pumping effectively. The absence of a pulse confirms that there is no circulation, meaning the heart has stopped functioning effectively, which is a hallmark of cardiac arrest.

In contrast, other options do not indicate cardiac arrest. For example, a severe headache could suggest many conditions, including migraines or tension headaches, but it does not specifically indicate that the heart has stopped beating. Pupils that are reactive to light suggest that there may still be some neurological function and are typically not associated with cardiac arrest, where you would expect unresponsiveness and possible fixed or dilated pupils. Localized chest pain may indicate a variety of issues, including angina or myocardial infarction, but again, does not confirm cardiac arrest, as the person could still have a pulse and be responsive. Thus, the combination of being unresponsive with no pulse unmistakably signifies that the patient has entered cardiac arrest, making it the correct response.