Final answer:
Prions are infectious proteins that replicate without nucleic acids and are associated with diseases like mad cow disease and Creutzfeldt-Jakob disease, leading to Prusiner receiving the Nobel Prize.
Step-by-step explanation:
Prions are best described as infectious proteins that cause disease by replicating their abnormally folded forms, which can accumulate in tissues, especially in the brain. Prions contain neither DNA nor RNA and are associated with several neurodegenerative disorders such as mad cow disease and Creutzfeldt-Jakob disease. This completely contradicts the once prevalent belief that all infectious agents must contain nucleic acids. The discovery by Stanley Prusiner regarding prions earned him the Nobel Prize, shifting the paradigm in our understanding of infectious diseases.
Therefore, the correct answer to the student's question is D) an infectious protein.