**Question:** Intra arterial injection of thiopentone causes –

A. Vasodilation
B. Vasoconstriction
C. Anesthesia
D. Increased blood pressure

**Core Concept:**
Thiopentone, also known as thiopental, is a barbiturate anesthetic drug used for inducing and maintaining general anesthesia. When administered, it affects various physiological processes, including the cardiovascular system.

**Why the Correct Answer is Right:**
Thiopentone works primarily as a central nervous system depressant, which leads to the suppression of the central chemoreceptors responsible for controlling breathing, heart rate, and blood pressure. In the context of intra arterial administration, this results in a more rapid onset and deeper anesthesia compared to intravenous administration.

**Why Each Wrong Option is Incorrect:**
A. Vasodilation (Option A) is incorrect because thiopentone, being a central nervous system depressant, actually causes vasoconstriction (narrowing of blood vessels) by inhibiting the release of vasodilatory substances like histamine, acetylcholine, and bradykinin.

B. Vasoconstriction (Option B) is also incorrect as mentioned above. Vasoconstriction is the expected physiological response to thiopentone administration due to its central nervous system depressant action.

C. Anesthesia (Option C) is a correct description of the main clinical effect of thiopentone, but it is not a direct result of intra arterial injection. The correct answer focuses on the physiological effects.

D. Increased blood pressure (Option D) is incorrect because thiopentone causes vasoconstriction, leading to a decrease in blood pressure, not an increase.

**Clinical Pearl:**
Intra arterial injection of thiopentone allows for the rapid onset of anesthesia by directly acting on the central nervous system. However, the primary focus should be on understanding the drug's effects on the cardiovascular system, as described above.

**Correct Answer:** B. Vasoconstriction

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In summary, the correct answer is vasoconstriction due to the central nervous system depressant effect of thiopentone. Although the drug eventually leads to anesthesia, the specific focus of the question is on the cardiovascular effects. The other options are incorrect for the following reasons:

1. Vasodilation (A) is the opposite of the expected physiological response to thiopentone.
2. Anesthesia (C) is a correct outcome but not a direct result of intra arterial injection.
3. Increased blood pressure (D) is the opposite of the actual effect of thiopentone, which causes vasoconstriction.
4. Vasoconstriction (B) is the correct answer because it explains the direct physiological effect of intra arterial administration of thiopentone.