What is a sign of anaphylaxis?

Anaphylaxis is a severe, potentially life-threatening allergic reaction that requires immediate medical attention. A hallmark sign of anaphylaxis is the combination of trouble breathing and swelling of the face. This occurs because anaphylaxis leads to the release of large amounts of histamine and other chemicals into the bloodstream, resulting in swelling of the airway, which can cause difficulty in breathing. The facial swelling often accompanies symptoms such as hives, itching, or redness as part of the body's systemic allergic response.

The other symptoms listed, such as headache, abdominal pain, and redness of the skin, can occur in various medical conditions, including mild allergic reactions, but they do not provide the same immediate danger to life associated with airway obstruction seen in anaphylaxis. Therefore, trouble breathing and swelling of the face clearly indicate the urgency of the situation and identify it as anaphylaxis, making this the correct choice.