Which of the following best describes a pathogen that would be considered a virus?

• A. An infectious protein that lacks nucleic acids and causes neurodegenerative diseases.
• B. A noncellular infectious agent composed of nucleic acid surrounded by a protein coat, requiring a host cell for replication.
• C. A unicellular organism with a cell wall and the ability to reproduce independently.
• D. A small, circular RNA molecule without a protein coat that infects plants.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the definition of a virus. A virus is a noncellular infectious agent composed of nucleic acid (either DNA or RNA) enclosed within a protein coat called a capsid. Viruses require a host cell to replicate because they lack the cellular machinery for independent reproduction.
2. Step 2: Analyze the first option: 'An infectious protein that lacks nucleic acids and causes neurodegenerative diseases.' This describes prions, which are infectious proteins but not viruses because they do not contain nucleic acids.
3. Step 3: Analyze the third option: 'A unicellular organism with a cell wall and the ability to reproduce independently.' This describes bacteria or other cellular organisms, which are not viruses because viruses are acellular and cannot reproduce independently.
4. Step 4: Analyze the fourth option: 'A small, circular RNA molecule without a protein coat that infects plants.' This describes viroids, which are infectious RNA molecules but lack a protein coat, so they are distinct from viruses.
5. Step 5: Conclude that the second option, 'A noncellular infectious agent composed of nucleic acid surrounded by a protein coat, requiring a host cell for replication,' best fits the definition of a virus.