**Core Concept**
The question is testing the clinical feature of ischaemia in the context of a supracondylar fracture of the humerus. Supracondylar fractures are a type of elbow fracture that occurs just above the condyles of the humerus, and ischaemia in this context refers to reduced blood flow to the affected limb. The clinical feature being tested is likely related to the pathophysiology of ischaemia and its effects on the limb.

**Why the Correct Answer is Right**
The earliest ischaemic feature after reduction of a supracondylar fracture is likely related to the reperfusion injury that occurs when blood flow is restored to the affected limb. This can lead to a condition known as "reperfusion syndrome," which is characterized by a sudden increase in blood flow and a subsequent release of inflammatory mediators and free radicals. This can cause a range of symptoms, including pain, swelling, and decreased sensation. The correct answer is likely related to one of these symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a specific symptom or feature of ischaemia. While pain is a common symptom of ischaemia, it is not the earliest feature.

**Option B:** This option is incorrect because it is not a direct consequence of ischaemia. Decreased sensation can be a result of ischaemia, but it is not the earliest feature.

**Option C:** This option is incorrect because it is not a specific symptom or feature of ischaemia. While oedema can be a result of ischaemia, it is not the earliest feature.

**Option D:** This option is incorrect because it is not a specific symptom or feature of ischaemia. While decreased capillary refill can be a sign of ischaemia, it is not the earliest feature.

**Clinical Pearl / High-Yield Fact**
One clinical pearl to remember is that reperfusion syndrome can occur after reduction of a supracondylar fracture, and it is characterized by a sudden increase in blood flow and a subsequent release of inflammatory mediators and free radicals. This can lead to a range of symptoms, including pain, swelling, and decreased sensation.

**Correct Answer:** D. Decreased capillary refill.