NBME 21 Synthesis of factor X (Stuart factor) (NBME Answers)

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Retired NBME 21 Answers

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A 3-year-old boy is brought to the emergency ...
Synthesis of factor X (Stuart factor) 🔍 / 📺 / 🌳 / 📖
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submitted by ∗mcl(671)

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Not sure if this is the right way to think about it, but if PT and PTT are both prolonged, this most likely means there is a problem with the common pathway (aka factor X).

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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 +2

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submitted by msw(4)

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theres a uworld question like this ... ,, rivoraxaban and other factor Xa inhibitors prolong pt and aPTT but they dont affect thrombin time ... while direct thrombin inhibitors prolong PT aPTT and Thrombin time , also unfractionated heparin prolongs aPTT and Thrombin Time (Theoreticallly it should prolong PT too but in the test the PT reagent contains heparin neutralizers that minimize this effect ) . sorry if this veered off course a little

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submitted by ∗agurl1000(5)

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Could someone explain to my why it is Factor X? Like I understand that PTT and PT are both elevated. I picked Factor X and then changed it to "hydrolysis of.." because I thought with a Factor X deficiency, wouldn't you have a prolonged thrombin time? I might be making this more complicated, but doesn't Factor X play a role in thrombin activation, so lack of it would lead to a prolonged thrombin time?

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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

agurl1000 THANK YOU SO MUCH! THIS MAKES SO MUCH SENSE! +

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!) +

https://www.google.com/search?q=what+does+thrombin+time+evaluate

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(1206)

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submitted by ∗usmle11a(102)

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i did this through elimination:

a,c and D intrinsic; ↑PTT only

B) which is actually the PIP2; involved in Gq signaling

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