What usually causes lymphadenitis?

Lymphadenitis is primarily caused by infection, making infection the correct choice. This condition occurs when the lymph nodes become swollen and inflamed, typically due to the body responding to an infection. Various infectious agents, including bacteria, viruses, and parasites, can lead to the development of lymphadenitis.

When the lymph nodes become involved in the immune response to these pathogens, they often increase in size, indicating that they are working to filter out harmful microorganisms and respond to the underlying infection. Common infections that lead to lymphadenitis can include upper respiratory infections, skin infections, and systemic infections like HIV or mononucleosis.

The other options—trauma to the lymph nodes, injuries to surrounding tissues, and autoimmune disorders—can potentially affect the lymphatic system or lymph nodes, but they are not the primary causes of lymphadenitis, which is chiefly associated with infectious processes.