**Core Concept**
Skeletal muscle tone is regulated by the central nervous system (CNS) through the action of neurotransmitters and neuromodulators. Various pharmacological agents can affect muscle tone by influencing the CNS, either directly or indirectly. In this context, we are examining the effects of different drugs on skeletal muscle tone.

**Why the Correct Answer is Right**
D-tubocurarin, a non-depolarizing neuromuscular blocking agent, acts by competitively inhibiting acetylcholine receptors at the neuromuscular junction, thereby blocking muscle contraction. However, its primary mechanism of action is not through CNS effects, but rather through direct blockade of the neuromuscular junction. This is in contrast to the other options, which all exert their effects through CNS mechanisms.

**Why Each Wrong Option is Incorrect**
**Option B:** Diazepam, a benzodiazepine, acts on the CNS by enhancing the effect of gamma-aminobutyric acid (GABA) at the GABA_A receptor, leading to sedation, muscle relaxation, and decreased muscle tone. Its mechanism is clearly CNS-mediated.
**Option C:** Baclofen, a GABA_B receptor agonist, also acts on the CNS to decrease muscle tone. By stimulating GABA_B receptors, it leads to a reduction in the release of excitatory neurotransmitters, resulting in muscle relaxation. Baclofen's effects are purely CNS-mediated.
**Option D:** Mephenesin is a skeletal muscle relaxant that acts on the CNS by increasing the release of gamma-aminobutyric acid (GABA) and decreasing the release of excitatory neurotransmitters, such as acetylcholine and glutamate. This results in muscle relaxation and decreased muscle tone, indicating a CNS-mediated effect.

**Clinical Pearl / High-Yield Fact**
When differentiating between skeletal muscle relaxants, it's essential to note that those with CNS effects, such as benzodiazepines and GABA_B receptor agonists, can lead to sedation and respiratory depression, particularly in high doses. In contrast, non-depolarizing neuromuscular blockers, like D-tubocurarin, primarily affect the neuromuscular junction without significant CNS effects.

**✓ Correct Answer: A. D-tubocurarin**