Which of the following is listed as a medical emergency?

An allergic reaction is categorized as a medical emergency because it can lead to serious and potentially life-threatening complications, such as anaphylaxis. This reaction can cause symptoms that impair breathing, swelling of the throat, and drastic drops in blood pressure, which requires immediate medical intervention.

In contrast, while the common cold, hypertension, and mild headache can be significant health issues, they typically do not require the urgent response associated with medical emergencies. The common cold is generally self-limiting and can often be managed with over-the-counter medications. Hypertension may require management, but unless it reaches a severe level leading to acute symptoms (like a hypertensive crisis), it usually is not an immediate threat. A mild headache can often be addressed with rest and hydration, making it not an emergency situation.