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nbme24/Block 4/Question#47

A 40-year-old man comes to the physician because of ...

HNPCC syndrome

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submitted by charlie(5),

I think there's a typo on that question. MSH2 gene mutation is the culprit for HNPCC. For MHS2 gene, according to what I just searched, causes Malignant Hyperthermia Susceptibility.

noplanb  Yeah I think it should be MSH +  
suckitnbme  Good to know I wasn't tripping out when I did this question +1  

 +3  upvote downvote
submitted by neonem(371),

This patient case sounds like he has iron deficiency anemia (anemia, low hematocrit, microcytic) from a GI bleed. To get this question right, you had to remember that the two major inherited GI cancer syndromes are FAP (due to mutation in APC gene, which is a tumor suppressor gene) and Lynch syndrome AKA hereditary non-polyposis colorectal carcinoma (HNPCC), caused by a mutation in a number DNA mismatch repair genes, of with MHS2 is a more common one.

The mechanisms of their carcinoma development are different; in FAP, tumors arise from a normal --> adenoma --> carcinoma sequence while in HNPCC, tumors arise from what's known as a microsatellite instability pathway, leading to spontaneous formation of a carcinoma (not preceded by a benign lesion like an adenoma)... You didn't need to know this to get this question right, but definitely good to know.

medpsychosis  To make it even simpler, if you narrowed it down to FAP vs HNPCC and looked at the image provided in the question, you'd see it's less likely to be FAP due to absence of numerous polyps which would be expected. So HNPCC would be your best choice! +2  
yb_26  I always get Li-Fraumeni and Lynch syndromes confused :/ +1  

 +0  upvote downvote
submitted by thotcandy(17),

I think there's something that hasn't been mentioned and people are over looking:

Patient is presenting with CRC + anemia which most commonly occurs with RIGHT sided cancer. And how does that cancer develop? Micro-satellite instability (vs left sided = obstruction and polyp-adenoma sequence).

Misat instab mostly commonly occurs in HNPCC due to mismatch repair defect.

Thus, even though there's a type in the gene, or even if you don't know the gene, there's enough info to figure out it's HNPCC.

FAP doesn't match the picture, and is a pylop-adenoma sequence cancer. Li-fraumeni is defect in p53 (which is the last stage of pylop-adenoma sequence so I assume it follows that too?).

The constipation is kinda tricky because you'd think obstruction but the big key here is ANEMIA due to CHRONIC BLEEDING.