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Retired NBME 20 Answers

nbme20/Block 1/Question#31 (reveal difficulty score)
A 66-year-old man has had fatigue, shortness ...
Focal pulmonary fibrosis ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
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submitted by โˆ—onyx(46)
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A radiographically visible air-fluid level suggests a pretty large lesion (hence, โ€œcavitaryโ€). That's not going to become normal tissue again. Six months following resolution of symptoms you can expect healing in the form of a scar; that is, fibrosis but only in a single spot.

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masonkingcobra  Robbin's: The basic mechanisms of fibrosis are the same as those of scar formation during tissue repair. However, tissue repair typically occurs after a short-lived injurious stimulus and follows an orderly sequence of steps, whereas fibrosis is induced by persistent injurious stimuli such as infections, immunologic reactions, and other types of tissue injury. The fibrosis seen in chronic diseases such as pulmonary fibrosis is often responsible for organ dysfunction and even organ failure. +5
stevenorange  What Differentiates Normal Lung Repair and Fibrosis? Basement membrane ! https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2645241/ +3
step1234  I'm a little confused here since FA20 209 states Klebsiella causes liquefactive necrosis in the lungs. Does the pulmonary fibrosis occur after the necrosis? +1
fatboyslim  @step1234 Yes pulmonary fibrosis occurs after liquefactive necrosis as a means of repair. Think of it as liquefactive necrosis = damage done in response to Klebsiella -> 6 months later you will see focal fibrosis in that area (scar formation) where the damage happened. +5
makingstrides  If there was an intact basement membrane, you would get normal pulmonary parenchyma. +1
skonys  Not granulomatous inflamm because thats indicative of TB. Not liquefactive nec (trick answer) because Klebsiella causes that actively and q stem says 6Mo post-resolution w/ abx. Not metaplastic pulmonary epithelium because metaplasia implies cellular adaptation to a stimuli (bronchial squamous metaplasia from cigarette smoke), not destruction entirely by liq-nec. IE: for cells to undergo metaplasia they need to be insulted but not destroyed entirely. Not normal pulmonary parenchyma because basement membrane/cytoskeleton architecture is inherently destroyed by cavitary lesion. +3



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