**Question:** Fetal haemoglobin has higher affinity for O2 due to

A. lower affinity for CO2
B. higher affinity for CO2
C. lower affinity for 2,3-diphosphoglycerate
D. higher affinity for 2,3-diphosphoglycerate

**Correct Answer:** D. higher affinity for 2,3-diphosphoglycerate

**Core Concept:**
Fetal haemoglobin (HbF) has higher affinity for O2 compared to adult haemoglobin (HbA). This difference in oxygen affinity is crucial for maintaining adequate oxygen delivery to the developing tissues while preventing hypoxia in the mother's blood. The primary molecular basis for this difference is the presence of the gamma-globin chains in HbF (alpha2 gamma2) compared to the adult haemoglobin (alpha2 beta2).

**Why the Correct Answer is Right:**
HbF has a higher affinity for O2 mainly due to the presence of the gamma-globin chains. The gamma-globin chains are more sensitive to the influence of 2,3-diphosphoglycerate (2,3-DPG). 2,3-DPG is an allosteric effector that affects the oxygen-binding affinity of haemoglobin. When 2,3-DPG levels increase, such as in acidosis, the oxygen-binding affinity of haemoglobin increases. In fetal haemoglobin, the presence of gamma-globin chains results in a higher affinity for 2,3-DPG, which in turn increases the oxygen affinity of fetal haemoglobin.

**Why Each Wrong Option is Incorrect:**

A. Lower affinity for CO2 is not related to the oxygen-binding affinity of haemoglobin. CO2 binding affects the oxygen-dissociation curve, but not the oxygen-binding affinity.

B. Higher affinity for CO2 would actually decrease the oxygen-binding affinity of haemoglobin, which is not consistent with the correct answer.

C. Lower affinity for 2,3-diphosphoglycerate would decrease the oxygen-binding affinity of haemoglobin, contrary to the correct answer.

D. Higher affinity for 2,3-diphosphoglycerate is the correct answer because it results in increased oxygen-binding affinity of fetal haemoglobin, which is crucial for maintaining adequate oxygen delivery in the developing fetus.

**Clinical Pearls:**
1. The higher oxygen affinity of fetal haemoglobin allows the fetus to maintain adequate oxygen delivery in the acidic environment of the uterus, where the acidosis reduces 2,3-DPG levels, thereby increasing the oxygen affinity of fetal haemoglobin.
2. The shift from fetal haemoglobin to adult haemoglobin occurs during the transition from fetal to adult life, which is regulated by various factors including fetal hypoxia, acidosis, and increased erythropoietin levels. This shift ensures a more efficient oxygen transport in the adult organism, where the oxygen affinity does not need to be as high as in the fetus.

3. 2,3-diphosphoglycerate is a crucial allosteric modulator that affects the oxygen-dissociation curve of haemoglobin