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Question 341
Physiology → Haematology
Classification
Question
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Following a small vascular injury, local vascular constriction and formation of a temporary platelet plug occur normally. Subsequent formation of an effective fibrin clot fails. Which interpretation best localises the defect?
Question Stem
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Options
A
Failure of the initial vascular response must have occurred
B
Platelet adhesion to collagen must have failed completely
C
Primary haemostasis cannot occur without prior fibrin polymerisation
D
Initial haemostasis is preserved, but the subsequent coagulation sequence is defective
E
The abnormality must be caused by loss of erythrocyte carbonic anhydrase
Explanation
Immediate haemostasis includes vascular constriction and formation of a temporary platelet plug. Coagulation subsequently generates thrombin and fibrin, with factor XIII and calcium contributing to formation of a stable fibrin mesh. Preserved vascular closure and platelet plugging with failure of effective fibrin clot formation therefore localises the abnormality to the subsequent coagulation process.
Option Validity
A) Local vascular constriction has occurred normally in the stem, so failure of the initial vascular response cannot explain the subsequent defect.
B) Formation of a temporary platelet plug indicates that primary platelet haemostasis has occurred rather than failed completely.
C) Platelets contribute to immediate haemostasis before formation of the stable fibrin mesh; primary haemostasis therefore does not require prior fibrin polymerisation.
E) Carbonic anhydrase participates in erythrocyte carbon dioxide handling and does not account for failure of the coagulation sequence.
Further Reading
Saxena R. Textbook for MRCOG-1. Chapter 3: Physiology – Haematology. pp.70–71.