This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Intra Uterine Growth Restriction, Intrapaum and Antepaum Fetal Surviellance, Subject name: Gynaecology & Obstetrics
The Kleihauer Betke test for detecting foetal erythrocytes is based on the fact that 
Select and explain. 
A. Adult erythrocytes are larger than those of foetus, B. HbA has higher oxygen affinity than HbF, C. HbF is more resistant acid elution than HbA, D. HbA takes up erythrosin stain less than HbF
C: HbF is more resistant acid elution than HbA
Answer: C: HbF is more resistant acid elution than HbA 
Explanation: The most commonly used quantitative test for fetal red cells in the maternal circulation is the acid elution or Kleihauer Betke test. Fetal erythrocytes contain HbF, which is more resistant to acid elution than HbA.