Which of the following symptoms indicates a severe allergic reaction?

Swelling of the throat is a critical symptom indicating a severe allergic reaction, commonly known as anaphylaxis. This condition can lead to airway obstruction, making it difficult to breathe, and requires immediate medical attention. When the throat swells, it can significantly impact airflow and oxygenation, which can be life-threatening if not treated promptly.

In contrast, increased appetite, headache, and nausea, while they can occur with various medical conditions or mild allergic reactions, do not inherently indicate the severity of an allergic response. They are more general symptoms that might relate to a range of issues, lacking the critical urgency associated with throat swelling in the context of an allergy. Understanding these distinctions is vital for recognizing potentially life-threatening scenarios.