What characteristic rash is associated with mononucleosis when treated with ampicillin?

In the context of mononucleosis, which is typically caused by the Epstein-Barr virus (EBV), the administration of ampicillin can lead to a specific type of rash. The characteristic polymorphous rash that appears is often maculopapular in nature but can exhibit varied forms and features across different areas of the skin.

This rash can be seen as part of an atypical reaction to the medication in patients with mononucleosis, and although some may confuse it with other forms of rashes due to different conditions or medications, it is notably associated with this presentation. The polymorphous nature indicates variability in the presentation, making it distinct in its appearance compared to other rashes.

Other types of rashes listed in the options, while potentially related to different etiologies or hypersensitivity reactions, do not specifically correlate with the treatment of mononucleosis with ampicillin in the way that a polymorphous rash does. This distinguishes the polymorphous rash as the correct response in this scenario.