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NBME 20 Answers

nbme20/Block 3/Question#18 (45.6 difficulty score)
An 18-year-old woman comes to the physician ...

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submitted by hayayah(1056),
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allmiiaF oaoaumstedn ssployoip si na laasotoum datnmnio Tanuodhss of sopply aeris tsaingtr retfa e;bptuyr onpa;clcino yswaal lnisevvo ct.uerm arcPyotilhcp mtylcceoo or lsee %100 pssregro ot CR.C

oaAmslout nindmoat dsesesai heva, no er,ageav 05% ncheac of gibne apdess nwod ot gff.rnsoip

sympathetikey  I would say this is Lynch Syndrome (APC is usually thousands of polyps) but lynch syndrome would generally have a family history of other cancers as well, so you might be right. Either way, both autosomal dominant so win win. +2  
smc213  uptodate states: Classic FAP is characterized by the presence of 100 or more adenomatous colorectal polyps +  
dickass  @sympathetikey Lynch Syndrome is literally called "Hereditary NON-POLYPOSIS colorectal cancer" +9  
fatboyslim  I think this actually is Lynch syndrome. Lynch syndrome can also develop colonic polyps but not nearly as bad as FAP. FAP has so many polyps you can't even see the normal mucosa. If you Google Lynch colonoscopy you can see that they develop a few polyps. +  
rockodude  I forgot it was AD inheritance but regardless at the time I was confused because APC is a tumor suppressor so it needs two hits. I guess AD inheritance and then you need another hit to develop CRC kind of like familial retinoblastoma or li fraumeni syndrome +  

submitted by cyrus_em(2),
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I 'tnod kwon ywh I feel the tocrcer rnwase is .001% Teh sueq ,assett w"ath si het ahcenc atht prfsgsniof liwl AELULTNEVY dlevoep cac?n,er ton niireth the mt"aniuot

rctpPohlycai omloyccte or lese %001 ULEYETVALN reossgpr to .CCR

lilyo  I also chose 100% with that same reasoning!!! +  
lilyo  @usmlecrasher, Yes it does so which is why if they inherited they have 100% chance of developing colon cancer later in life and the question was confusing because it didnt ask what is the percentage that this patient will have children with this mutation? It might be my language barrier but I don't know. +  
diegolc26  Its a little tricky bk they want you to think the probability of developing cancer IF the offspring HAS the mutation, which is 50%. I mean, he has 50% chances of inheriting the cancer developing mutation. Thats how I think they want to trick us. +1  
mbourne  @cryus_em, you're putting way too much emphasis on the word "eventually". Either way the question and answer makes sense. It's an AD disease, so the chance of the offspring inheriting it would be 50% (and if they inherit it, they WILL develop adenocarcinoma of the colon, so the answer is still 50%). +1