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NBME 18 Answers

nbme18/Block 1/Question#33 (reveal difficulty score)
A 50-year-old man is admitted to the hospital ...
Long half life of factor II (prothrombin) ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–

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 +8  upvote downvote
submitted by โˆ—aakb(41)

warfarin inhibits epoxide reductase which prevents gamma carboxylation of NEW vitamin K dependent clotting factors. the therapeutic efficacy of warfarin is delayed until prexisting/OLD factors get consumed, which usu takes at least 3 days.

Therefore, it will not affect the PT for at least three days because the old factors are still around. Since factor II has the longest half life, it takes the longest for the old factor II to go away/get used up and that is why the PT has not increased yet in this patient.

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mariame  I am confused, he has had already 2 days with Heparin, and Heparin enhances anti thrombin which inhibits thrombin. +2
an1  @mariame they're asking why the PT is normal. PT is in reference to warfarin more than it is to heparin (refer to PTT). The reason Heparin is given is because it'll have rapid effects, but long term we want to use warfarin. Protein C and S (Anti-coagulants) are lost first, but the coagulant factors already in the blood remain (7 goes first, 2 goes last). warfarin only affects formation of new factors, not ones already made +

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submitted by โˆ—anechakfspb(77)

Factor II (prothrombin) has the longest half life (FA 2019 p. 405)

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lokotriene  FA2020 p413 +2

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submitted by โˆ—eghafoor(3)

Kicking myself rn for getting this wrong, but here's how to think of this (for anyone who needs a step-by-step approach instead of a page number):

(1) Heparin inhibits Factor IIa, and factors Xa by potentiating Antithrombin III---more specifically, it binds up Factor IIa (thrombin) and prevents it from activating fibrinogen into fibrin; it also binds up Factor Xa (which indirectly also decreases the formation of IIa)

(2) The fact that they threw in Warfarin in the question stem is a distractor. Warfarin's effects typically take a while for them to be seen, which is evidenced by the fact that the PT & INR are normal in this case--even after 24 hours post-warfarin administration

(3) This means the effects of the Heparin are still going on, which could only mean that factor IIa is still being inhibited by ATIII and thus, hella low = PTT is prolonged

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