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USMLE Step 1 Microbiology — practice by topic

Browse 41 questions across 1 Microbiology topics. Choose what you want to revise, then open a question to check its answer and worked solution.

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Microbiology topics

Virology (41)

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What is a virion?Read the answer and worked solution →Which imaging method is the only one that reveals the internal structures of a virion?Read the answer and worked solution →The protein subunits that build up a viral capsid carry which name?Read the answer and worked solution →Which hypothesis about the origin of viruses proposes that they descend from free-living cells?Read the answer and worked solution →

Worked practice questions

What is a virion?

  1. A host cell that has just been lysed
  2. A protein subunit that builds a capsid
  3. A viral gene sitting in host DNA
  4. A single virus particle outside a cell
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Correct answer: D. A single virus particle outside a cell

1. A virion is one individual virus particle. 2. It is the infectious form the virus takes while it is outside a host cell. 3. A capsid subunit is a capsomere, and viral DNA inside a host genome is a prophage, so neither of those is a virion. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 21 "Viruses", section 21.1 Viral Evolution, Morphology, and Classification_

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Which imaging method is the only one that reveals the internal structures of a virion?

  1. Scanning electron microscopy
  2. Light microscopy at high power
  3. Transmission electron microscopy
  4. Either light or scanning microscopy
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Correct answer: C. Transmission electron microscopy

1. Both scanning and transmission electron microscopy show the surface structure of virions. 2. Only transmission electron microscopy shows what lies inside the particle. 3. A light microscope cannot resolve viruses at all, apart from some large poxvirus virions. 4. Scanning electron microscopy stops at the outer surface, so it is the trap answer here. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 21 "Viruses", section 21.1 Viral Evolution, Morphology, and Classification_

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The protein subunits that build up a viral capsid carry which name?

  1. Ribosomes
  2. Nucleotides
  3. Capsomeres
  4. Glycoproteins
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Correct answer: C. Capsomeres

1. Every virion carries a nucleic acid genome inside a protective protein layer called the capsid. 2. That capsid is assembled from repeated protein subunits, and each subunit is a capsomere. 3. Glycoproteins sit on the surface for attachment, and ribosomes belong to cells rather than to viruses. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 21 "Viruses", section 21.1 Viral Evolution, Morphology, and Classification_

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