**Core Concept**
NMDA (N-methyl-D-aspartate) blockers are a class of medications that inhibit the action of glutamate at NMDA receptors. In the context of antitussives, NMDA blockers are used to suppress coughing by modulating the neurotransmitters involved in the cough reflex.

**Why the Correct Answer is Right**
The correct answer is a medication that blocks the action of NMDA receptors to suppress coughing. By inhibiting the NMDA receptors, this medication reduces the excitatory neurotransmission involved in the cough reflex, thereby providing an effective antitussive effect. The NMDA receptor is a subtype of glutamate receptor that plays a crucial role in the excitatory neurotransmission in the central nervous system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not an NMDA blocker used as an antitussive.
**Option B:** This option is incorrect because it is not a medication that blocks NMDA receptors to suppress coughing.
**Option C:** This option is incorrect because it is not an NMDA blocker used as an antitussive, although it may have some other pharmacological effects.

**Clinical Pearl / High-Yield Fact**
NMDA blockers can also have analgesic and anxiolytic effects, making them useful in treating conditions such as chronic pain and anxiety disorders.

**Correct Answer:** A. Dextromethorphan