A patient attends for the extraction of a grossly carious lower left third molar tooth. Which instrument is likely to be used to protect the lingual nerve?

The correct choice of instrument to protect the lingual nerve during the extraction of a lower left third molar tooth is the periosteal elevator. This instrument is specifically designed to reflect soft tissue from bone and gain access to the tooth for extraction while providing protection to surrounding structures, including nerves like the lingual nerve, which is located close to the lower third molar.

Using a periosteal elevator allows the dental professional to carefully lift the mucoperiosteum away from the bone, ensuring that the nerve is not inadvertently injured during the extraction process. This is crucial as the lingual nerve supplies sensation to the tongue and floor of the mouth, and damage to it can result in complications, such as altered sensation or numbness in those areas.

The other instruments listed serve different purposes that are not focused on nerve protection during such a procedure. A barbed broach is primarily used for root canal treatment to remove pulp tissue, a Mitchell's trimmer is designed for shaping and finishing dental restorations, and a rotary paste filler is used in restorative dentistry to fill cavities but does not serve a protective role during extractions. This distinction highlights the specific functionality of the periosteal elevator in maintaining patient safety during dental surgical procedures.