Which of the following is an infectious protein and is simpler than a virus?

• A. Prion
• B. Bacteriophage
• C. Retrovirus
• D. Viroid

Answer: (A)

Verified step by step guidance

1. Understand the definition of an infectious protein: An infectious protein is a misfolded protein that can induce other normal proteins to also misfold, leading to disease without involving nucleic acids like DNA or RNA.
2. Recall the characteristics of the options: Bacteriophages are viruses that infect bacteria and contain nucleic acids; Retroviruses are RNA viruses that use reverse transcriptase; Viroids are small infectious RNA molecules without a protein coat.
3. Identify which option fits the description of being an infectious protein and simpler than a virus: Prions are infectious proteins that lack nucleic acids and are indeed simpler than viruses.
4. Confirm that prions cause diseases by protein misfolding, such as Creutzfeldt-Jakob disease, distinguishing them from viruses and viroids which rely on nucleic acids.
5. Conclude that the correct answer is 'Prion' because it matches the criteria of being an infectious protein and simpler than a virus.