Which general anaesthetic selectively inhibits excitatory NMDA receptors:
**Core Concept**
The question is testing knowledge of the pharmacology of general anaesthetics and their mechanism of action on excitatory neurotransmission, specifically the NMDA receptor subtype. NMDA receptors are a subtype of glutamate receptors that play a crucial role in excitatory neurotransmission in the central nervous system.
**Why the Correct Answer is Right**
Ketamine is a general anaesthetic that selectively inhibits excitatory NMDA receptors by blocking the glycine binding site, which is essential for NMDA receptor activation. This action results in a decrease in excitatory neurotransmission, leading to anaesthesia and analgesia. Ketamine's mechanism of action is thought to be responsible for its dissociative anaesthetic effects, which include altered perception, hallucinations, and amnesia.
**Why Each Wrong Option is Incorrect**
**Option A:** Propofol is a general anaesthetic that inhibits GABA_A receptors, leading to sedation and anaesthesia. It does not selectively inhibit NMDA receptors.
**Option B:** Midazolam is a benzodiazepine that also inhibits GABA_A receptors, leading to sedation and amnesia. It does not selectively inhibit NMDA receptors.
**Option C:** Fentanyl is an opioid analgesic that acts on mu-opioid receptors to produce analgesia. It does not selectively inhibit NMDA receptors.
**Clinical Pearl / High-Yield Fact**
Ketamine has a unique property of producing dissociative anaesthesia, which can be useful in certain clinical situations, such as managing acute trauma or reducing pain in patients with opioid tolerance.
**Correct Answer: C. Ketamine**