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NBME 20 Answers

nbme20/Block 1/Question#33 (44.3 difficulty score)
A 17-year-old girl comes to the emergency ...

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 +6  upvote downvote
submitted by strugglebus(154),
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O,K so fi I rebmemre ocytercrl tsih si the one thta hswso hte enaitenihrc ntt.rpea oratolidnhimc is alos edssap by eht erthom; oheer,wv ti anc vaeh laabrevi rstyiesveipx dan tleociepmn ep,ceeanntr cwhhi si hwy msoe meesbmr ewer not deae.cfft

hyoscyamine  Also, question said there was a deficiency in NADH dehydrogenase activity which is another fancy way of saying complex I in the mitochondria. +13  
yotsubato  That unaffected male really threw me off... : ( +19  
charcot_bouchard  It was pure MELAS description. the unaffected male threw me off +2  
mbourne  I think the affected male on the right side is actually a helpful hint. Mitochondrial conditions can be inherited by males or females, but are only passed on through the females. +  

 +6  upvote downvote
submitted by h0odtime(42),

Severity of mitochondrial diseases is directly related to the proportion of abnormal to normal mitochondria within patient cells.

Heteroplasmy having different mitochondrial genomes in a single cell.

Homoplasmy in contrast, is seen in healthy tissue where mitochondrial DNA is identical.

UWorld explanation ID 1935. For those visually inclined.

The most likely mode of inheritance of the disorder in this patient can be found by using the inheritance algorithm:

  • Does offspring with disease have a parent with disease? (Y/ N)

    • if YES dominant (does not skip generations)

      • is there male-to-male transmission of disease? (Y/ N)

        • if YES autosomal dominant

        • if NO: do daughters of affected male have disease? (Y/ N)

          • if YES X-linked dominant

          • if NO Mitochondrial

 +3  upvote downvote
submitted by dickass(78),

Got this one wrong because of those 2 unaffected children. Here's the explanation from FA pg 59:

Mitochondrial inheritance - Transmitted only through the mother. All offspring of affected females may show signs of disease.

Variable expression in a population or even within a family due to heteroplasmy.

Heteroplasmy basically means that multiple mitochondria are transmitted to each offspring. Their ratio may change at each generation, and cause more severe or less severe disease, even within the same family.

Those unaffected dudes lucked out.

dickass  That'll teach me not to skim FA, you really gotta look up the words you don't know. +  
chandlerbas  heteroplasmy is a fancy way of saying variable expressivity just specific to mitochondrial diseases i do declare +  

Easy AF explanation for doing mitoch. q's: Mothers have diseased children; Fathers don't

Dont forget this trick.