Which of the following is a common sign of an adverse reaction?

Itching, also known as pruritus, is a common sign of an adverse reaction, particularly in response to medications or allergens. This reaction occurs as part of the body's immune response, where histamines are released by mast cells in the skin, leading to irritation and the sensation of itch. This type of response is often linked to allergic reactions, indicating that the body is reacting to a substance it perceives as harmful.

Headache, fatigue, and dry mouth can also occur due to various reasons, including medication side effects or other health conditions, but they are not as specifically indicative of an adverse reaction compared to itching. While these symptoms can be associated with certain types of adverse reactions, itching is more specifically related to allergic responses and skin reactions, making it a clearer indicator of a potential adverse effect.