**Core Concept**
Carisoprodol is a centrally acting muscle relaxant used to treat muscle spasms and pain. It works by acting as a GABA_A receptor agonist, which leads to sedative and muscle relaxant effects. However, carisoprodol itself is not the active metabolite responsible for its therapeutic effects.

**Why the Correct Answer is Right**
Carisoprodol is metabolized in the liver by the enzyme CYP2C19 to form its active metabolite, meprobamate. Meprobamate is an anxiolytic agent that acts as a GABA_A receptor agonist, contributing to the muscle relaxant effects of carisoprodol. This metabolite is responsible for the therapeutic effects of carisoprodol.

**Why Each Wrong Option is Incorrect**
**Option A:** Mephenytoin is an antiepileptic medication that is not a metabolite of carisoprodol.
**Option B:** Meprobamate is indeed the correct answer, so we skip this one.
**Option C:** Diazepam is a benzodiazepine that is not a metabolite of carisoprodol but rather a separate class of muscle relaxant.

**Clinical Pearl / High-Yield Fact**
Meprobamate, the active metabolite of carisoprodol, has a potential for abuse and dependence, which is a significant clinical consideration when prescribing carisoprodol.

**Correct Answer: B. Meprobamate**