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NBME 20 Answers

nbme20/Block 3/Question#18

An 18-year-old woman comes to the physician to ...


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submitted by hayayah(525),

Familial adenomatous polyposis is an autosomal dominant mutation. Thousands of polyps arise starting after puberty; pancolonic; always involves rectum. Prophylactic colectomy or else 100% progress to CRC.

Autosomal dominant diseases have, on average, 50% chance of being passed down to offspring.

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. +1  
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" +5  

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submitted by cyrus_em(0),

I don't know why I feel the correct answer is 100%. The ques states, "what is the chance that offsprings will EVENTUALLY develop cancer?, not inherit the mutation"

Prophylactic colectomy or else 100% EVENTUALLY progress to CRC.

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. +  
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%). +