Which of the following symptoms is commonly associated with tonsillitis?

Fever is a common symptom of tonsillitis due to the body's immune response to the infection or inflammation of the tonsils. When the tonsils become infected, either by a viral or bacterial source, the body reacts by raising its temperature as part of the immune defense mechanism. This increase in body temperature helps create an environment less conducive to the growth of pathogens.

Tonsillitis typically presents with other symptoms such as sore throat, difficulty swallowing, and swollen lymph nodes, but a fever is a key indicator of the body's fight against the infection. This makes it a classic symptom that healthcare professionals use to diagnose tonsillitis.

Other options like insomnia, nausea, and headache may be associated with general illness or other types of infections but are not specific symptoms of tonsillitis. Fever serves as a critical sign linked directly to the inflammatory process occurring in the tonsils.