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Question 339
Physiology → Haematology
Classification
Question
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A freshly collected blood sample contains normal platelets and coagulation factors, but free calcium ions are removed. Which consequence most directly follows?
Question Stem
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Options
A
Only the extrinsic pathway is prevented
B
Only platelet adhesion to collagen is prevented
C
Clotting is prevented despite the presence of the other coagulation components
D
Fibrin formation proceeds but bleeding time becomes prolonged
E
Prothrombin is converted to thrombin without prothrombin activator
Explanation
Calcium ions are essential for blood clotting. Prothrombin activator converts prothrombin to thrombin in the presence of calcium, and calcium also participates with factor XIII in formation of a stable fibrin mesh. Removal of calcium therefore prevents clotting.
Option Validity
A) Calcium is required in the coagulation sequence and its removal is not confined to inhibition of the extrinsic pathway.
B) Platelet adhesion to exposed collagen involves pseudopodia, but removal of calcium is specifically described as preventing blood clotting.
D) Calcium is required before completion of stable clot formation; its removal therefore cannot be characterised merely as prolonging bleeding time while fibrin formation proceeds normally.
E) Conversion of prothrombin to thrombin requires prothrombin activator and calcium; removal of calcium does not permit this reaction to proceed independently.
Further Reading
Saxena R. Textbook for MRCOG-1. Chapter 3: Physiology – Haematology. p.71.