**Core Concept**
Thiopentone is a barbiturate used for inducing general anesthesia, and its accidental intravascular injection can cause severe complications due to its vasodilatory and cardiac depressant effects. The rapid onset of action and the high concentration of thiopentone in the arterial blood can lead to severe consequences.

**Why the Correct Answer is Right**
The correct answer is D. Pain because thiopentone injection into an artery causes severe pain due to the direct irritation of the arterial wall. This is because thiopentone has a high pH and can cause chemical irritation of the arterial endothelium, leading to pain. The pain is often described as severe and can be accompanied by a sensation of burning or stinging. The pain is usually transient but can be a warning sign of potential complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Analgesia is not a symptom of thiopentone injection into an artery, as thiopentone is an anesthetic agent that can cause analgesia when used properly, but not in this context.

**Option B:** Paralysis is not a direct symptom of thiopentone injection into an artery, although high doses of thiopentone can cause muscle relaxation and paralysis. This is not the immediate effect of accidental injection into an artery.

**Option C:** Skin ulceration is not a symptom of thiopentone injection into an artery. While thiopentone can cause skin necrosis and ulceration when extravasated, this is not the first symptom of accidental injection into an artery.

**Clinical Pearl / High-Yield Fact**
When administering thiopentone, it is essential to use a large vein and to monitor the patient closely for signs of accidental intravascular injection, including pain, which can be a warning sign of potential complications.

**✓ Correct Answer: D. Pain**