**Core Concept**
Intravenous anaesthetics are a class of medications administered directly into a vein to induce or maintain general anaesthesia. These agents work by modulating neurotransmitter activity in the central nervous system, specifically targeting GABA receptors, NMDA receptors, or other ion channels.

**Why the Correct Answer is Right**
The correct answer is a medication that does not belong to the intravenous anaesthetic class. Intravenous anaesthetics typically act by enhancing the inhibitory effects of GABA, the primary inhibitory neurotransmitter in the brain, or by blocking excitatory neurotransmission. Examples include propofol, which binds to GABA_A receptors, and ketamine, which blocks NMDA receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** Propofol is an intravenous anaesthetic agent that works by enhancing GABA_A receptor activity, leading to sedation and anaesthesia. It is not the correct answer.
**Option B:** Ketamine is another intravenous anaesthetic agent that blocks NMDA receptors, inducing dissociative anaesthesia. It is not the correct answer.
**Option C:** Midazolam is a benzodiazepine that acts as an intravenous anaesthetic agent by enhancing GABA_A receptor activity. It is not the correct answer.
**Option D:** Fentanyl is an opioid analgesic, not an intravenous anaesthetic agent. It is used for pain management but does not induce general anaesthesia.

**Clinical Pearl / High-Yield Fact**
When using intravenous anaesthetics, it's essential to monitor patients closely, as these agents can cause respiratory depression, hypotension, and other adverse effects.

**Correct Answer:** D. Fentanyl.