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Thursday, April 26, 2007

Vitamin K is High Yield for the USMLE Step 1

  • Vit. K is needed for the γ-carboxylation of clotting factors 2, 7, 9 and 10
    • This gives them a Ca binding site
    • This explains why the vit. K dependent factors are the same factors that are dependent on calcium
  • Deficiency:

o Primary problem is an inability to γ-carboxylate factors 2, 7, 9 and 10

      • therefore, factors ARE made but can NOT be activated by Ca
    • Involves several factors, including factor 7 which:
      • has the shortest t½ of all the clotting factors
        • therefore, factor 7 is the first factor unable to be activated
      • Since factor 7 is specific to the extrinsic pathway the extrinsic pathway is affected first
      • Since the prothrombin time (PT) is a measure of the extrinsic pathway, it is the value expected to be elevated first
        • Leads to ↑↑PT (AND eventually ↑PTT) – therefore, PT is most sensitive since its ↑’d 1st
        • Bleeding time, a measure of platelet function, is normal since platelets are not affected
    • Deficiency can be seen in the following patients:
      • Newborns born at home
        • Breast milk has low levels of vit K
        • All babies in the US are given vit. K shots @ birth in the hospital to prevent hemorrhagic disease of the newborn
      • Pts on broad spectrum antibiotics
        • Bacteria in colon synthesize much of our vit. K
      • Pts w/ steatorrhea
        • Loss of fat in the stools also leads to a loss of the fat soluble vitamins, including vit. K
      • Vitamin E toxicity
        • Inhibits vitamin K dependent carboxylation of clotting factors
  • Treatment
    • Vitamin K Injection
    • If bleeding is serious then give fresh frozen plasma


Amit said...

really nice. Just finished reviewing the coag cascade so this makes sense.

Michelle G. said...

Now THAT was nicely explained. Needed that. Aren't you glad your generosity of time & effort are still helping people years later?

liz said...

thank you very much from a med student down under! :)

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