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Discussion Starter · #1 ·
A 32-year-old nulliparous female with a history significant for SLE and two miscarriages presents with localized tenderness of gradual onset, with erythema, along the path of a superficial vein in her foot. Although the symptoms are indicative of superficial thrombophlebitis, her PT is within normal limits and her PTT is prolonged. An antiphospholipid antibody syndrome is suspected, and she is treated empirically with indomethacin while a mixing test is ordered. Which of the following reactions in the coagulation cascade occurs through cofactors bound to phospholipids?
  • Factor VII → Factor VIIa
  • Factor IX → Factor IXa
  • Factor X → Factor Xa
  • Prothrombin (II) → Thrombin (IIa)
  • All of the above
 

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Why?

Can you please explain as to why is it "all of the above"? I was thinking that since PT is normal Factor 7 is normal...but I guess I don't really understand this question. Good question though!:sorry:
 

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Discussion Starter · #6 ·
Can you please explain as to why is it "all of the above"? I was thinking that since PT is normal Factor 7 is normal...but I guess I don't really understand this question. Good question though!:sorry:
I think you are trying to answer a question that was never asked! :) This happens to me all the time! :rolleyes: I read a qbank question, and about halfway through it, my mind is busy solving the clinical problem. I want to look down to see what the answer choices are to confirm my suspicions... fatal mistake! :toosad: I have skipped the last sentence or two of the stem, which is not asking what I wanted it to ask.

In this case, you probably saw "normal PT and abnormal PTT" and started thinking of which factors would be affected, then cast your eyes over the answer choices. In fact, the question was only asking which activations required phospholipids, as the PTT is phospholipid-dependent. The activation of II, VII, IX, X (and Dulbee adds VIII and XII to our list) occur through cofactors bound to phospholipids on the platelet and blood vessel endothelial surface in a calcium-coordination complex. Factor XI is also a serine protease, but apparently works differently. :shrugs...:

The rest of the explanation would say that in this patient an antiphospholipid antibody syndrome is suspected, possibly lupus anticoagulant, and a mixing test is ordered to see if 20% normal plasma corrects the PTT, which would rule out LA. If the mixing test is still prolonged, phospholipid-sensitive coagulation assays like activated partial thromboplastin time (aPTT) and dilute Russell viper venom time (RVVT) would rule in LA. We would also screen for anti-cardiolipin antibodies.

hope that helps...
 
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