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Question 337
Physiology → Haematology
Classification
Question
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A patient with pernicious anaemia has markedly enlarged individual erythrocytes. Despite the increased cell size, the haematocrit is reduced. Which mechanism best reconciles these findings?
Question Stem
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Options
A
Large erythrocytes necessarily trap less plasma during centrifugation
B
Total red-cell mass is reduced despite the increased size of individual erythrocytes
C
Macrocytosis causes a compensatory increase in plasma clotting factors
D
The buffy coat occupies a greater proportion of the haematocrit
E
Macrocytic erythrocytes become less dense than plasma
Explanation
Haematocrit can fall in macrocytic megaloblastic anaemia such as pernicious anaemia because total red-cell mass is decreased despite the large size of individual erythrocytes. Cell size alone therefore does not determine the packed-cell volume.
Option Validity
A) Trapped plasma explains why centrifuged haematocrit is slightly higher than a calculated value; it does not explain the fall in haematocrit in pernicious anaemia.
C) Plasma clotting factors do not provide the explanation for the reduced haematocrit in macrocytic megaloblastic anaemia.
D) The buffy coat consists of white blood cells and platelets and does not explain the reduced red-cell proportion in this setting.
E) Haematocrit measurement depends on the packed proportion of red cells; the relevant explanation is reduced total red-cell mass despite increased individual cell size.
Further Reading
Saxena R. Textbook for MRCOG-1. Chapter 3: Physiology – Haematology. p.68.