C. FVIII
D. Prothrombin Correct answer = A. Antithrombin III (ATIII) inhibits the action of thrombin (FIIa), a Gla-containing protein of clotting that activates the extrinsic, intrinsic, and common pathways. Excretion of ATIII in nephrotic syndrome allows the actions of FIIa to continue, resulting in a hypercoagulable state. The other choices are proteins required for clotting. Their excretion in the urine would decrease clotting.
5.12. Blocking the action of which of the following proteins would be a rational therapy for hemophilia B?
A. FIX
B. FXIII
C. Protein C
D. Tissue factor pathway inhibitor
Correct answer = D. Hemophilia B is a coagulopathy caused by decreased thrombin production by the common pathway as a result of a deficiency in FIX of the intrinsic pathway. Because the extrinsic pathway also can result in thrombin production, blocking the inhibitor of this pathway (tissue factor pathway inhibitor) should, in principle, increase thrombin production.
5.13. The parents of a newborn baby girl refuse to allow the baby to be given the injection of vitamin K that is recommended shortly after birth to prevent vitamin K deficiency bleeding, which is caused by the low levels of the vitamin in newborns. The activity of which one of the following protein factors involved in clotting would be decreased with vitamin K deficiency?
A. FV
B. FVII
C. FXI
D. FXII
E. FXIII
Correct answer = B. FVII is a γ-carboxyglutamate (Gla)-containing protein of clotting. The creation of Gla residues by γ-glutamyl carboxylase requires vitamin K as a coenzyme. FII, FIX, and FX, as well as proteins C and S that limit clotting, also contain Gla residues. The other choices do not contain Gla residues.
5.14. Thrombin, produced in the common pathway of clotting, has both procoagulant and anticoagulant activities. Which one of the following is an anticoagulant activity of thrombin?
A. Activating FXIII
B. Binding to thrombomodulin
C. Increasing nitric oxide production
D. Inhibiting FV and FVIII
E. Inhibiting platelet activation
F. Inhibiting tissue factor pathway inhibitor
Correct answer = B. Thrombin bound to thrombomodulin activates protein C that degrades the accessory proteins FV and FVIII, thereby inhibiting clotting. Activation of FXIII by thrombin strengthens the fibrin clot. Nitric oxide, a vasodilator made by endothelial cells, decreases clot formation. It is not affected by thrombin. Thrombin is a powerful activator of platelets. Inhibition of tissue factor pathway inhibitor would increase clotting.
5.15. A student is reviewing the use of prothrombin time (PT) and activated partial thromboplastin time (aPTT) in evaluating a suspected deficiency of a clotting protein. Which one of the following results would be correct for a deficiency in FXIII?
A. Both prothrombin time and activated partial thromboplastin time are decreased.
B. Both prothrombin time and activated partial thromboplastin time are increased.
C. Both prothrombin time and activated partial thromboplastin time are unchanged.
D. Only prothrombin time is affected.
E. Only activated partial thromboplastin time is affected.