Which of the following conditions is a sign of an allergic reaction during a physical examination of the head?

Angioedema is a condition characterized by the rapid swelling of the area beneath the skin, often affecting the face, lips, and sometimes the throat. It is commonly associated with allergic reactions, particularly those triggered by medications, foods, or insect stings. In the context of a physical examination, observing angioedema would raise suspicion of an allergic response, as it is a direct manifestation of the body's exaggerated immune response to allergens.

The other conditions listed do not indicate an allergic reaction. Sinus tenderness may suggest sinusitis or other non-allergic conditions affecting the sinuses. A normal head size would indicate no immediate issues related to swelling or other conditions, while ataxia refers to a lack of voluntary coordination of muscle movements, which can be related to neurological conditions rather than allergies. Thus, angioedema is the clear sign of an allergic reaction during a physical examination of the head.