MLT GlobeVirology: General virology & lab methods
76 Virology MCQs on General virology & lab methods with answers and explanations.
A laboratory wants to isolate a suspected virus from a clinical sample. Which system is unsuitable?
Viruses are obligate intracellular parasites and cannot multiply on cell-free media such as nutrient agar; they need living cells, eggs or animals.
A researcher isolates viruses that lyse multidrug-resistant bacteria for phage therapy. These viruses are called:
Bacteriophages are viruses that infect and replicate within bacteria. Plasmids are extrachromosomal DNA, prions are infectious proteins, viroids infect plants.
Virus-infected host cells release a protein that induces antiviral resistance in neighbouring cells. This protein is:
Type I interferons (alpha/beta) are secreted by infected cells and trigger antiviral states in nearby cells.
Most medically important viruses fall within which size range?
Viruses range from roughly 20 nm (parvoviruses) to about 300 nm (poxviruses), below light-microscope resolution. Micrometre sizes belong to bacteria.
Which structural element does every virus possess, whether naked or enveloped?
All virions have a capsid enclosing the genome. Envelopes and spikes occur only in enveloped viruses, and tail fibres mainly in some bacteriophages.
Many "spherical" viruses such as adenovirus actually have a polyhedral capsid. What geometric shape is it?
Icosahedral symmetry, with 20 triangular faces, is the most economical closed shell built from identical subunits and is the usual polyhedral capsid form.
Which statement about virus structure and replication is FALSE?
Transcription produces mRNA; translation of that mRNA on ribosomes produces proteins. The other statements are correct.
To titrate a bacteriophage stock, dilutions are mixed with host bacteria in soft agar. What is counted to give the titre?
Each plaque arises from one infectious phage, so plaque counts give plaque-forming units per mL. EM counts total, not infectious, particles.
A virology lab uses HeLa and Vero cells that can be subcultured indefinitely. What kind of culture are these?
Continuous (immortal) lines are transformed or tumour-derived and divide indefinitely. Primary cultures survive one or two passages; diploid strains about 50 divisions.
A patient has fever and rash; the laboratory must decide whether a viral infection is current or past. Which antibody class indicates recent (primary) infection?
IgM is the first antibody of the primary response and declines within weeks to months, so specific IgM indicates recent infection. IgG persists and marks past exposure or immunity.
Which of these viruses has a DNA genome?
Adenoviruses carry linear dsDNA. Rhabdoviruses, rubella virus and enteroviruses are all RNA viruses.
Which immunoglobulin, secreted in dimeric form into saliva, tears and intestinal fluid, chiefly protects mucosal surfaces against viruses?
Secretory IgA, a dimer joined by J chain and carrying secretory component, neutralises viruses at mucosal surfaces such as the respiratory and intestinal tracts.
Which of these viruses has a DNA genome?
Poxviruses are large, complex dsDNA viruses replicating in the cytoplasm. Retroviruses (including HIV), picornaviruses and influenza are RNA viruses.
What term describes a complete, infectious virus particle outside a host cell?
A virion is the complete extracellular particle (genome, capsid and any envelope). Prions are proteins, viroids are naked RNA, and a provirus is integrated viral DNA.
Which step of the viral replication cycle mainly determines which host species and tissues a virus can infect?
Tropism is largely set by binding of viral attachment proteins to specific cell-surface receptors, e.g. HIV gp120 to CD4. Later steps do not define host range as strongly.
Starting from one double-stranded target molecule and assuming perfect efficiency, how many copies exist after the second PCR cycle?
Each cycle doubles the target: 1 becomes 2 after cycle 1 and 4 after cycle 2 (2ⁿ). Eight copies would require three cycles.
Which innate lymphocytes kill virus-infected cells early, recognising cells that have lost MHC class I, before specific T cells appear?
NK cells detect "missing self" (reduced MHC I) and stress ligands on infected cells and lyse them within days. Neutrophils mainly target bacteria.
A researcher wants to see the capsid structure of a virus and the internal ultrastructure of a bacterium. Which instrument is required?
Viruses (20-300 nm) and cell ultrastructure are below the ~200 nm resolution of light microscopes; electron microscopy resolves them.
Which virus is NOT recognised as a sexually transmitted pathogen?
Rubella spreads by respiratory droplets and transplacentally. HBV, HIV and HSV-2 are all transmitted sexually.
Laboratory diagnosis of viral infection relies on several approaches. Which is NOT one of them?
Viral diagnosis uses detection of genome (PCR), antigen, antibody (serology) or isolation in culture. A blood count may give clues but does not identify a virus.