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NBME 22 Answers

nbme22/Block 2/Question#31 (reveal difficulty score)
An unimmunized 1-year-old boy is admitted to ...
Phase variation 🔍 / 📺 / 🌳 / 📖

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 +26  upvote downvote
submitted by yotsubato(1208)

In biology, phase variation is a method for dealing with rapidly varying environments without requiring random mutation. It involves the variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial population. As such the phenotype can switch at frequencies that are much higher (sometimes >1%) than classical mutation rates. Phase variation contributes to virulence by generating heterogeneity. Although it has been most commonly studied in the context of immune evasion, it is observed in many other areas as well and is employed by various types of bacteria, including Salmonella species.

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whoissaad  is it the same thing as antigenic variation? +12
dorsomedial_nucleus  No, antigenic variation involves genomic rearrangement Phase variation can be thought of as MORE or LESS of something. An on/off switch. No DNA is being rearranged, just under or overexpressed in response to the environment. +8
makinallkindzofgainz  This isn't in Zanki, Lightyear, or First Aid, and I don't remember ever learning about this in class. Thanks NBME! :D +21
jurrutia  You wouldn't expect the difference in phenotype of a given organism in a given patient to change because of mutations. It has to be something other than mutation. Phase variation is the only option that sounded like non-mutation. +
l0ud_minority  One of my lectures at my school covered this apparently. In one of their 150+ slide power point handouts small font on slide 65 bottom left hand corner. +1
ankigravity  Don't know if it was in UWorld a year ago, but there is a UWorld question about phase variation. I made card from it stating, "{{c1::Phase}} variation refers to a reversible switch between an “all-or-none” (on/off) expressing phase, resulting in variation in the level of expression of one or more proteins between individual cells of a clonal population." +1
abhishek021196 Phase Variation - Phase variation is a reversible process that helps bacteria to quickly generate diversity and adapt to rapidly changing environments. One mutation in a 'phase variable' gene can switch expression ON, and another mutation in the same part of DNA will switch the expression of the gene back OFF. DON'T KNOW WHAT DIFFERENCE BETWEEN ANTIGENIC AND PHASE VARIATION THOUGH? +

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