What are prions composed entirely of?

Prions are infectious agents that are composed entirely of proteins. They are misfolded forms of normal proteins found in the brain. The unique characteristic of prions is that they can induce abnormal folding in normal proteins, leading to a chain reaction of misfolding that disrupts normal cellular function. This abnormal folding is associated with several neurodegenerative diseases, such as mad cow disease and Creutzfeldt-Jakob disease.

Unlike viruses, which contain genetic material (either DNA or RNA) and require a host cell to replicate, prions do not have any nucleic acids and replicate by causing misfolding of existing proteins in the host. Therefore, the correct answer emphasizes the distinctive nature of prions as purely protein-based infectious agents.