What condition may cause loss of the nail if untreated and usually involves pus at the nail base?

The condition characterized by the loss of the nail if untreated, often accompanied by pus at the nail base, is indeed onychia. Onychia refers to an inflammation of the nail matrix, which can result from infections. When the nail matrix becomes infected, it leads to the formation of pus and, ultimately, can cause the nail to detach or fall off if the condition is not addressed properly.

This inflammation and infection can arise from various sources, including bacterial infections or trauma to the nail. If left untreated, the ongoing irritation and infection can severely impact the health of the nail and its growth, resulting in significant nail loss.

Other conditions mentioned, such as onychocryptosis (ingrown nail), onychophagy (nail biting), and pterygium (a condition involving the skin growing onto the nail), while related to nail issues, do not typically present with pus at the nail base or lead to nail loss in the same manner as onychia does. Therefore, onychia is the correct choice due to its specific characteristics of inflammation and potential complications.