Which of the following agents is a known NMDA receptor antagonist used as an anesthetic?

Ketamine is recognized as an NMDA receptor antagonist that is commonly used as an anesthetic. Its mechanism involves blocking the N-methyl-D-aspartate (NMDA) receptor in the central nervous system, which plays a critical role in pain perception, memory, and neuroplasticity. This blockade leads to dissociative anesthesia, characterized by a trance-like state, pain relief, and amnesia, making it particularly useful in procedures where analgesia and sedation are required without the patient being fully unconscious.

In contrast, other agents listed do not primarily function as NMDA receptor antagonists. Propofol acts mainly as a GABA agonist, enhancing inhibitory neurotransmission, which leads to sedation and anesthetic effects. Sevoflurane, a volatile anesthetic, works by facilitating inhibitory neurotransmission through various mechanisms but does not specifically antagonize NMDA receptors. Thiopental is a barbiturate that also enhances GABA activity but does not target NMDA receptors in its anesthetic action.

Thus, the unique role of ketamine as a specific NMDA receptor antagonist sets it apart from the other options, affirming its selection as the correct answer in this context.