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NBME 21 Answers

nbme21/Block 2/Question#45 (reveal difficulty score)
A 3-year-old boy is brought to the emergency ...
Synthesis of factor X (Stuart factor) 🔍 / 📺 / 🌳

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submitted by keycompany(344),
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nCa yeomobsd esaelp iaxlnpe how oiiinngnzon ainarotid ahs an iniizngo ffte.ce

submitted by mcl(646),
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toN user fi hsti is eth irght ayw to ithnk boaut ti, tub fi TP and TPT are bhto nlep,grdoo sthi tmos kyllei aemns heetr is a bomrelp twih eth nmcoom pywthaa ak(a tofcar .X)

temmy  exactly...i just thought the problem has to be where they meet or somewhere similar to both..hence the common pathway 12(PTT Heparin) 7 (PT, Warfarin) 11 9 10 5 2 1 In my head, both sides are looking for the perfect 10 +1  

submitted by msw(3),
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ehrtes a wroudl uotiqens ikel isht ... ,, inaarrabxov dan orteh oratcf Xa binoitishr olrogpn pt nad TaTP tub yeht ntdo fatcef rmnitobh temi ... ielwh tedirc hnbrmoti sibohnriit lopongr TP TaTP dan mTrhnoib etim , soal afnodautnitcer rniepah roglonsp aTPT nad Thromnib eiTm lihe(Toetrylalc ti hlsodu oglrnop PT oto tub ni eth stet teh PT ereagnt onniacts pareihn tnruaesriezl tath izeinimm isth cfefte ) . syrro if sthi edever off uresco a eittll

submitted by agurl1000(5),
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Culdo neemoso lianxpe ot my yhw ti si Foatrc X? eLki I sanetddnru tath TPT and TP ear btho tle.adeve I cpekdi atFroc X adn htne dchagen it ot ihly"osydsr f.."o casebeu I uotthgh thiw a tarcoF X diicfcn,eey wut'onld oyu heav a drnogelpo toihmnrb ?emit I migth eb gkamin htis emor peai,dlctomc ubt ds'toen orctFa X alpy a rloe ni bntirhmo taociinva,t os lkca fo it ulodw adel to a lodrgpone ntihormb imet?

drdoom  a person with FXD (factor x deficiency) will have "prolonged thrombin time" inside their body (in vivo), yes, but their THROMBIN_TIME (the lab test) will be completely normal +1  
drdoom  when they run the lab test, what they do is just add a bunch of lab-grown (recombinant?) ACTIVATED thrombin to your plasma and then count the seconds until a clot forms. that’s your THROMBIN_TIME. notice how if you have fucked up Factor X, it doesn't matter because the lab guy is adding NORMAL, lab-grown/off-the-shelf activated thrombin to your plasma sample. +1  
drdoom  in other words, by adding the lab thrombin, we "circumvent" any problems leading up to AND INCLUDING the production of functional thrombin. (since perfectly functional thrombin is what’s being supplied by the lab guy.) THUS what THROMBIN_TIME (the test) is really asking is, “Is there anything inside this patient that is interfering with the conversion of fibrinogen->fibrin?” +1  
drdoom  SO, what the test should REALLY be called is not THROMBIN_TIME (which is confusing as hell) but TIME_REQUIRED_TO_MAKE_FIBRIN_FROM_FIBRINOGEN +1  
drdoom  ☺️☺️~ +  
drdoom  p.s. I did a little more reading and it seems like thrombin is derived from plasma donors (not made recombinantly in a dish😁 sorry!) +

The thrombin_time evaluates that part of the hemostatic process where soluble fibrinogen is changed into fibrin threads. It measures the time required for a fibrin clot to form following the addition of a standard amount of thrombin to plasma.

(emphasis mine)

+3/- drdoom(1113),

submitted by usmle11a(94),
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i ddi hsti ghrohut tni:lieanmio

,ca dna D irinic;tsn PT↑T lony

B) wchih si tyluclaa het ;PI2P livonedv in qG nsiigglan

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