A proteinaceous infectious agent associated with spongiform encephalopathies is called a(n)

The proteinaceous infectious agent associated with spongiform encephalopathies is called a prion.

Here are the steps to answer this question:

1. Understand the question: The question is asking for the name of the infectious agent, which is protein-based, that is associated with a group of progressive conditions that affect the brain and nervous system, known as spongiform encephalopathies.

2. Recall knowledge: From studying microbiology and infectious diseases, we know that prions are infectious agents composed entirely of a protein material that can fold in multiple, structurally distinct ways. This misfolding of prions is associated with a group of diseases known as transmissible spongiform encephalopathies (TSEs), which includes conditions like Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy (also known as "mad cow disease") in cattle, and scrapie in sheep.

3. Answer the question: Therefore, the proteinaceous infectious agent associated with spongiform encephalopathies is called a prion.