Which of the following is a sign of a serious allergic reaction?

Swelling of the throat and difficulty breathing is a clear sign of a serious allergic reaction, commonly known as anaphylaxis. This medical emergency can occur when an individual is exposed to an allergen they are highly sensitive to, leading to the rapid onset of severe symptoms. The swelling in the throat can obstruct the airway, which can restrict airflow and pose a significant risk to life if not treated promptly. Difficulty breathing is another critical symptom associated with this severe response, necessitating immediate medical attention.

In contrast, itching and sneezing, nausea and vomiting, and headache and fatigue may occur in various less severe allergic reactions or other conditions, but they do not typically signal the urgency or life-threatening nature of anaphylaxis. While these symptoms can indicate an allergy, they do not carry the immediate danger that swelling of the throat and trouble breathing does.