## **Core Concept**
The question tests the knowledge of anaesthetic agents and their side effects, specifically the association with postoperative hallucinations. Postoperative hallucinations can be a complication of various factors including the anaesthetic agents used. Understanding the properties and side effects of different anaesthetic agents is crucial for anaesthesiologists.

## **Why the Correct Answer is Right**
Ketamine is known for its dissociative properties and can cause vivid dreams, hallucinations, and a sense of detachment from reality. It acts on the central nervous system by blocking NMDA receptors, which are involved in the regulation of glutamate, a neurotransmitter associated with memory and learning. This blockade can lead to the dissociative effects and hallucinations observed with ketamine use.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Propofol is a commonly used intravenous anaesthetic agent known for its rapid onset and short duration of action. It is not typically associated with causing hallucinations; instead, it is often used for sedation and has a favourable profile for quick recovery.
* **Option B:** Midazolam is a benzodiazepine used for sedation, inducing sleepiness or relaxation before diagnostic or surgical procedures. While it can cause amnesia, it is less likely to cause hallucinations compared to ketamine.
* **Option D:** Fentanyl is a potent opioid analgesic used for pain management and as part of anaesthesia. Opioids can cause euphoria and dysphoria but are not commonly associated with causing hallucinations directly.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that ketamine can cause **emergence reactions** including hallucinations, confusion, and vivid dreams. This is a critical consideration when choosing anaesthetic agents, especially in patients with a history of psychiatric disorders.

## **Correct Answer:** C. Ketamine.