**Core Concept**
NMDA receptors are a subtype of glutamate receptors, which play a crucial role in synaptic plasticity, memory formation, and learning. They are ligand-gated ion channels that allow for the influx of calcium ions into the postsynaptic neuron.

**Why the Correct Answer is Right**
NMDA receptors are composed of multiple subunits, including NR1, NR2, and NR3. The NR1 subunit is essential for the receptor's function and is the primary binding site for the antagonist ketamine. The other options, however, do not describe NMDA receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** This could be a description of AMPA receptors, another subtype of glutamate receptors, which are ligand-gated ion channels but do not contain the NR2 subunit.
**Option B:** This might describe kainate receptors, a third subtype of glutamate receptors that are also ligand-gated ion channels but have distinct pharmacological properties.
**Option C:** This could be a description of GABA receptors, which are ligand-gated ion channels but are involved in inhibitory neurotransmission rather than excitatory neurotransmission like NMDA receptors.

**Clinical Pearl / High-Yield Fact**
NMDA receptor antagonists, such as ketamine and memantine, are used in the treatment of certain types of pain and as an adjunct in the treatment of major depressive disorder.

**Correct Answer: A. AMPA receptors. AMPA receptors are a subtype of glutamate receptors, but they are not NMDA receptors.**