Which symptom can be indicative of CNS conditions according to the provided information?

The correct answer focuses on the symptom of loss of hearing, which can be indicative of central nervous system (CNS) conditions. Hearing is closely tied to neurological functions, and changes in auditory perception can point to underlying CNS issues such as tumors, infections, or demyelinating diseases that affect the pathways associated with sensory processing.

The other symptoms, while they may indicate various medical conditions, are less specifically tied to CNS issues. Mild swelling can be localized and does not typically relate to CNS function directly. Joint pain generally suggests musculoskeletal or inflammatory conditions, and deep pain under the sternum may relate to gastrointestinal or cardiovascular issues rather than CNS conditions. Therefore, loss of hearing serves as a more direct sign of potential CNS involvement.