An individual is admitted to the hospital displaying significant psychological and sensory distortions. A neurological scan reveals that a specific type of receptor, normally activated by the brain's primary excitatory neurotransmitter, is being blocked. Based on this mechanism, which substance is the most likely cause of the individual's symptoms?

A new psychoactive substance is found to cause significant perceptual distortions. Laboratory analysis reveals that its primary effect is to bind to a specific type of glutamate receptor, preventing the neurotransmitter from activating it. Based on this mechanism, which of the following existing substances does this new drug most closely resemble in its action?