**Core Concept**
Thiopentone is a barbiturate used for induction of anesthesia, and its intra-arterial injection can cause arterial spasm. To prevent this complication, a specific dose of thiopentone is used when administered intra-arterially.

**Why the Correct Answer is Right**
When thiopentone is injected intra-arterially, a dose of 25-50 mg is recommended to prevent arterial spasm. This is because the lower dose helps to minimize the risk of vasospasm, which can lead to ischemia and tissue damage. The mechanism of action is thought to be related to the vasodilatory properties of thiopentone, which helps to counteract the vasoconstrictive effects of the barbiturate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a dose of 100 mg would be excessive and increase the risk of arterial spasm.
**Option B:** This option is incorrect because a dose of 10 mg would be too low to effectively prevent arterial spasm.

**Clinical Pearl / High-Yield Fact**
When administering thiopentone intra-arterially, it is essential to use the recommended dose to prevent arterial spasm and minimize the risk of complications.

**Correct Answer:** D.