Protein-only infectious agents – Neurodegeneration: Which agent is associated with transmissible neurodegenerative diseases in livestock and humans?

Introduction:
Transmissible spongiform encephalopathies (TSEs) such as scrapie in sheep, bovine spongiform encephalopathy in cattle, and Creutzfeldt–Jakob disease in humans are unusual infectious neurodegenerative disorders. The causative agent defies classical definitions of life and infection. This question probes recognition of the unique agent class involved.

Given Data / Assumptions:

• TSEs show long incubation, spongiform neuropathology, and lack of inflammation.
• Agents are resistant to nucleic acid–targeting methods and many disinfectants.
• Transmission has been documented in experimental and some natural contexts.

Concept / Approach:
The prion hypothesis identifies the infectious agent as an abnormal, misfolded isoform of a host protein (PrP^Sc) that templates conformational change in normal PrP^C. Therefore, prions—not viruses, viroids, or hypothetical entities—are implicated in TSEs.

Step-by-Step Solution:
Step 1: Match TSEs with protein-only infectivity.Step 2: Exclude viroids (small circular RNA in plants) and virions (general term for virus particles).Step 3: Recognize “virino” as a speculative concept lacking consensus.Step 4: Conclude prions are the correct agents.

Verification / Alternative check:
Biochemical purification and resistance patterns support the proteinaceous nature of the agent; genetic studies link PRNP mutations to familial prion diseases, reinforcing the mechanism.

Why Other Options Are Wrong:

• Viroids: plant pathogens; nucleic acid–based.
• Virions: any complete virus particle; not specific to TSEs.
• Virinos: hypothetical; not established.
• Satellites: subviral agents requiring helper viruses; unrelated to TSEs.

Common Pitfalls:
Assuming all infectious agents must contain nucleic acid; conflating prions with viruses due to transmissibility.

Final Answer:
Prions.