Which of the following is a sign of a possible allergic reaction in a patient?

A swollen tongue is a significant indicator of a possible allergic reaction, particularly in the context of anaphylaxis or other hypersensitivity responses. Allergic reactions can lead to an inflammatory response that causes various symptoms, including swelling in the mucous membranes and soft tissues, such as the tongue. This condition, known medically as angioedema, can obstruct the airway, making it a critical symptom that requires immediate attention.

In contrast, the other options represent different medical issues that are not typically associated with allergic reactions. A dry socket generally occurs after a tooth extraction when a blood clot fails to form or is dislodged, leading to pain and inflammation in the extraction site. Red painful lesions are often indicative of infections or skin conditions such as herpes simplex or shingles but do not specifically point to an allergy. A fungal infection is caused by the proliferation of fungal organisms and is not related to the body's allergic response. Thus, a swollen tongue stands out as a key sign of an allergic reaction that healthcare professionals must recognize and respond to promptly.