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Question 349
Physiology → Haematology
Classification
Question
SSBAQuestion Header
A coagulation abnormality selectively involves a factor assessed by the extrinsic pathway test rather than the intrinsic pathway test.
Question Stem
Which factor most specifically fits this pattern?
Options
A
Factor VIII
B
Factor IX
C
Factor VII
D
Factor XI
E
Factor XII
Explanation
PT assesses the extrinsic and common pathways and is prolonged by abnormalities of factors VII, X, V and II. APTT assesses the intrinsic and common pathways and is prolonged by deficiency or inhibition of factors II, V, VIII, IX, X, XI and XII. Factor VII is therefore the option specifically associated with the extrinsic-pathway test.
Option Validity
A) Factor VIII abnormality prolongs APTT and reflects involvement of the intrinsic pathway.
B) Factor IX is included among factors whose deficiency or inhibition prolongs APTT.
D) Factor XI abnormality is detected through prolongation of APTT rather than PT.
E) Factor XII belongs to the group of factor abnormalities associated with prolonged APTT.
Further Reading
Saxena. Textbook for MRCOG-1. Physiology - Haematology - Prothrombin Time and Activated Partial Thromboplastin Time, p.72.