**Core Concept**
Dissociative anaesthesia is a type of anaesthesia characterized by a cataleptic state, with patients often remaining conscious but unresponsive to pain and their surroundings. This phenomenon is mediated through the blockade of NMDA receptors in the brain, which are crucial for the integration of sensory information.

**Why the Correct Answer is Right**
The correct answer is Ketamine. Ketamine works by blocking NMDA receptors in the brain, leading to a disconnection between sensory input and higher-level cognitive processing. This results in a state of dissociation, where patients may remain conscious but are unable to perceive or respond to their environment. Additionally, ketamine's action on NMDA receptors also contributes to its analgesic and sympathomimetic effects.

**Why Each Wrong Option is Incorrect**
* **Option A:** Thiopentone is a barbiturate-based induction agent that produces general anaesthesia through GABA receptor potentiation, but it does not produce dissociative anaesthesia.
* **Option B:** Propofol is a short-acting sedative-hypnotic agent that works through GABA receptor potentiation, leading to general anaesthesia, but it does not have dissociative anaesthetic properties.
* **Option C:** Midazolam is a benzodiazepine that produces sedation and amnesia through GABA receptor potentiation, but it does not produce dissociative anaesthesia.

**Clinical Pearl / High-Yield Fact**
Ketamine's dissociative anaesthetic effects are also associated with a reduction in the release of acetylcholine, which can lead to a decrease in salivation and an increase in heart rate.

**Correct Answer: D. Ketamine.**