Vitamin K deficiency
Started by medman88
You are right. Petechiae are more of a sign of a bleeding time, not PT or aPTT, issue. However that doesn't mean that they couldn't occur with vitamin K deficiency, but they are definitely not reflective of it, as you've mentioned.
I'm also not 100% sure how far along you are with your studies at this point, but here are a few things to be aware of:
Vitamin K2 is inactive. Vitamin K1 is active. Warfarin inhibits the K2 --> K1 conversion by inhibiting vitamin K epoxide reductase. You already know that.
However, don't confuse that enzyme with the one that vitamin K is a cofactor for, which is gamma glutamyl carboxylase. That enzyme name isn't written in FA, although it's more just implied, but I've seen it come up in practice questions, and they'll throw the epoxide reductase in there as a distractor.
With regard to what I've encountered in practice questions, if there's bleeding in a newborn, immediately think vitamin K deficiency or factor XIII deficiency. Neonates have sterile intestines, so they require vitamin K immediately after birth, so that deficiency is the most common, particularly if there is "bleeding from the umbilical stump." I had encountered factor XIII deficiency in a Kaplan QBook question. Apparently that's fairly common too, and it presents like vitamin K deficiency in a neonate.
In contrast, factor XII, not XIII, deficiency results in pro-coagulation because Hageman factor (factor XII) converts prokallikrein to kallikrein, and kallikrein converts plasminogen to plasmin. So without factor XII, there's pro-coagulation (or anti-anti-coagulation).
I've also seen vitamin K as an answer choice to an easy question, but it was listed as phytonadione, so be aware that that's the medical name for vitamin K1. It's also called phylloquinone, but I've only seen the former show up so far, not this latter one.
And the other thing to know is that factor VII has the shortest half-life of any of the factors, so that's why vitamin K deficiency prolongs PT, not aPTT, first.
Last thing, I had remembered from MS1 that parsley is the best known source of vitamin K, in addition to the cruciferous vegetables (which you can remember by saying "BBCC"), which include broccoli, Brussell sprouts, cauliflower and cabbage, however, in terms of practice questions, green leafy vegetables almost always points to folate, not vitamin K. The USMLE is overly obsessed with intestinal flora as being the vitamin K association.
Hope that helps,
Great stuff thanks! However this is the first I've heard of factor XIII def being a big issue. Is there anything specific that I should know about it other then what you said, also was wondering where you found that if you remember. Great call on the medical name of Vit K1 btw had no idea about that.
Kaplan QBook
