ASCP Coagulation 3

Question 1

Which anticoagulant mechanism does antithrombin (AT) primarily use?

Accepted Answer

Serine protease inhibition potentiated by heparin

Answer

Antithrombin is a serine protease inhibitor (serpin) that irreversibly inhibits thrombin (IIa), Factor Xa, IXa, XIa, and XIIa. Heparin binding induces a conformational change in AT that accelerates its inhibitory activity approximately 1000-fold.

Question 2

A mixing study showing 'no correction' (aPTT remains prolonged after mixing with normal plasma) is consistent with:

Accepted Answer

Factor VIII inhibitor

Answer

Factor deficiencies correct on mixing because normal plasma provides the missing factor. Inhibitors (antibodies against clotting factors) do not correct because the antibody neutralizes the factor provided by normal plasma.

Question 3

Platelet function analyzer (PFA-100) closure time is prolonged in all of the following EXCEPT:

Accepted Answer

Hemophilia A (Factor VIII deficiency)

Answer

PFA-100 tests primary hemostasis (platelet plug formation under shear stress). Hemophilia A is a secondary hemostasis (coagulation cascade) defect — the platelet plug forms normally, so PFA-100 closure time is normal.

Question 4

Rivaroxaban (Xarelto) is a direct oral anticoagulant (DOAC) that specifically targets:

Accepted Answer

Factor Xa

Answer

Rivaroxaban is a direct Factor Xa inhibitor. It binds the active site of Factor Xa without requiring antithrombin as a cofactor, blocking conversion of prothrombin to thrombin.

Question 5

A neonate presents with purpura fulminans and severely low protein C levels. This presentation is caused by:

Accepted Answer

Homozygous or compound heterozygous protein C deficiency

Answer

Neonatal purpura fulminans is caused by homozygous or compound heterozygous protein C deficiency (protein C activity below 1%), resulting in uncontrolled microvascular thrombosis affecting skin, brain, and eyes immediately after birth.

ASCP Practice Test

ASCP Practice Test

ASCP Coagulation 3