**Core Concept**
The oxyhaemoglobin dissociation curve is a graphical representation of the relationship between the partial pressure of oxygen (pO2) and the saturation of haemoglobin with oxygen. The P50 value represents the partial pressure of oxygen at which 50% of haemoglobin is saturated with oxygen.

**Why the Correct Answer is Right**
The P50 value is an indicator of the affinity of haemoglobin for oxygen, with lower values indicating a higher affinity and higher values indicating a lower affinity. In adults, the normal P50 value is approximately 3.6 kPa, reflecting a balance between the need for sufficient oxygen delivery to tissues and the need to maintain adequate oxygenation of the blood. This value is influenced by various factors, including pH, temperature, and the presence of 2,3-bisphosphoglycerate (2,3-BPG) in red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** 1.8 kPa is too low, indicating an abnormally high affinity of haemoglobin for oxygen, which would result in excessive oxygen binding and potentially lead to tissue hypoxia.
**Option B:** 2.7 kPa is also too low, indicating an increased affinity of haemoglobin for oxygen, which would be seen in conditions such as cyanotic congenital heart disease or chronic hypoxia.
**Option D:** 4.5 kPa is too high, indicating a decreased affinity of haemoglobin for oxygen, which would result in reduced oxygen binding and potentially lead to tissue hypoxia.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the P50 value shifts in response to changes in pH, with an increase in P50 (i.e., a decrease in affinity) in acidotic conditions and a decrease in P50 in alkalotic conditions. This is an important consideration in critically ill patients with acid-base disturbances.

**✓ Correct Answer: C. 3.6 kPa**