**Core Concept**
Tramadol is a centrally acting analgesic that works primarily by inhibiting the reuptake of norepinephrine and, to a lesser extent, serotonin. This action enhances the release of these neurotransmitters, leading to increased pain modulation.

**Why the Correct Answer is Right**
Tramadol's mechanism of action involves the inhibition of norepinephrine and serotonin reuptake by the enzyme norepinephrine transporter (NET) and the serotonin transporter (SERT), respectively. This results in an increase in the concentration of these neurotransmitters in the synaptic cleft, which binds to their respective receptors, ultimately leading to analgesia. Tramadol also has a weak agonist action on the μ-opioid receptor, although this is not its primary mechanism of action.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe tramadol's primary mechanism of action.
**Option B:** This option is incorrect as tramadol does not work primarily through GABA inhibition.
**Option C:** This option is incorrect as tramadol is not a direct agonist of the NMDA receptor.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that tramadol has a ceiling effect, meaning that there is a maximum dose beyond which increasing the dose will not provide additional analgesia.

**Correct Answer: C. A weak μ-opioid receptor agonist.