**Core Concept**
Ketamine is a dissociative anaesthetic agent that primarily works by blocking NMDA receptors in the brain, resulting in a disconnection between the thalamus and the cortex. Unlike other anaesthetic agents, ketamine has a unique pharmacological profile that makes it less likely to cause hypotension on injection.

**Why the Correct Answer is Right**
Ketamine's mechanism of action involves increasing sympathetic tone through stimulation of the locus coeruleus, which leads to increased heart rate and blood pressure. This contrasts with other anaesthetic agents, such as thiopentone and propofol, which can cause vasodilation and a subsequent decrease in blood pressure. Halothane, an inhalational anaesthetic, also causes vasodilation and can lead to hypotension. However, ketamine's sympathomimetic effects help to counteract these effects, making it less likely to cause hypotension.

**Why Each Wrong Option is Incorrect**
**Option B:** Propofol causes vasodilation and a decrease in systemic vascular resistance, leading to a significant drop in blood pressure on injection.
**Option C:** Halothane causes vasodilation and a decrease in cardiac contractility, resulting in a decrease in blood pressure.
**Option D:** Thiopentone causes vasodilation and a decrease in cardiac contractility, leading to a decrease in blood pressure on injection.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ketamine's sympathomimetic effects can also lead to increased salivation, bronchial secretions, and an increased heart rate, which can be beneficial in certain clinical scenarios but require careful monitoring.

**Correct Answer Line**
✓ Correct Answer: A. Ketamine