**Core Concept**
Haloperidol, a typical antipsychotic, can cause extrapyramidal side effects (EPS), including akathisia, dystonia, and parkinsonism, due to its high affinity for dopamine D2 receptors in the nigrostriatal pathway. This blockade disrupts the normal balance between dopamine and acetylcholine, leading to motor symptoms.

**Why the Correct Answer is Right**
To treat haloperidol-induced dystonia, an anticholinergic agent is used to counteract the excessive cholinergic activity caused by dopamine blockade. Benztropine mesylate is a centrally acting anticholinergic that can effectively alleviate dystonic symptoms by increasing the availability of acetylcholine in the synaptic cleft.

**Why Each Wrong Option is Incorrect**
**Option A:** None of the other options are correct, as they do not address the specific issue of haloperidol-induced dystonia.
**Option B:** Ondansetron, an antiemetic, would not be effective in treating dystonia.
**Option C:** Valproate, an anticonvulsant, may have some efficacy in treating agitation, but it is not the preferred choice for treating dystonia.

**Clinical Pearl / High-Yield Fact**
When treating patients with antipsychotics, it's essential to be aware of the risk of EPS and consider adding anticholinergic agents prophylactically, especially in high-risk patients.

**Correct Answer: C. Valproate is incorrect - the correct answer is actually Benztropine, but it was not listed, so the question is invalid.