SBA — Single Best Answer · Review
Question 360
Physiology → Haematology
Classification
Question
SSBAQuestion Header
A patient has an isolated deficiency of factor VIII with preserved initial haemostasis.
Question Stem
Which coagulation-test pattern is most consistent with the affected pathway?
Options
A
Prolonged PT with normal APTT
B
Prolonged bleeding time with normal clotting time
C
Normal APTT because factor VIII belongs to the extrinsic pathway
D
Prolonged PT because factor VII activation depends on factor VIII
E
Prolonged APTT with no requirement for PT prolongation from the factor VIII deficiency itself
Explanation
APTT assesses the intrinsic and common coagulation pathways and is prolonged by deficiency or inhibition of factor VIII. PT assesses the extrinsic and common pathways and is prolonged by abnormalities including factors VII, X, V and II. Factor VIII deficiency therefore principally produces an intrinsic-pathway abnormality detected by APTT.
Option Validity
A) Factor VIII is assessed through the intrinsic pathway and therefore prolongs APTT rather than selectively prolonging PT.
B) Factor VIII deficiency is a coagulation-factor defect; initial haemostasis and bleeding time can remain normal.
C) Factor VIII belongs to the intrinsic rather than extrinsic coagulation pathway and its deficiency prolongs APTT.
D) PT begins with activation of factor VII and assesses the extrinsic and common pathways; isolated factor VIII deficiency is not described as causing PT prolongation.
Further Reading
Saxena. Textbook for MRCOG-1. Physiology - Haematology - Activated Partial Thromboplastin Time and Haemophilia A, p.72.