Which of the following is a sign of an allergic transfusion reaction?

A sign of an allergic transfusion reaction is typically characterized by itching, which is a symptom associated with the release of histamine and other mediators in response to allergens. When a patient is allergic to components of the blood product being transfused, the immune system reacts, leading to symptoms that often include localized or generalized itching, hives, or rash.

Although fever, hypotension, and coughing can occur in various types of transfusion reactions, they are not specific indicators of an allergic reaction. Fever may indicate a febrile non-hemolytic transfusion reaction or infection, hypotension could suggest a more severe reaction such as an anaphylactic response or hemolytic reaction, and coughing might be observed in cases of transfusion-related acute lung injury (TRALI) or fluid overload, but they do not specifically indicate an allergic reaction per se. Itching, on the other hand, is a direct manifestation of the allergic response, making it the most appropriate choice to indicate an allergic transfusion reaction.