**Core Concept**
Metoclopramide-induced dystonic reactions are a common side effect of this antiemetic drug, which can cause involuntary muscle contractions. The drug of choice for treating this adverse effect is an anticholinergic agent that can counteract the dopamine-blocking effects of metoclopramide.

**Why the Correct Answer is Right**
Promethazine is a phenothiazine derivative with significant anticholinergic properties, which makes it an effective treatment for metoclopramide-induced dystonic reactions. By blocking the action of acetylcholine at muscarinic receptors, promethazine can counteract the dopamine-blocking effects of metoclopramide, thereby reducing muscle contractions. This is particularly useful in treating acute dystonic reactions, which can be a life-threatening emergency.

**Why Each Wrong Option is Incorrect**
**Option A:** Pheniramine is an antihistamine with some anticholinergic activity, but it is not the preferred choice for treating metoclopramide-induced dystonic reactions due to its lesser efficacy compared to promethazine.
**Option C:** Chlorpromazine is a first-generation antipsychotic with significant dopamine-blocking effects, which would exacerbate the dystonic reaction rather than treating it.
**Option D:** Prochlorperazine is another dopamine-blocking agent that can cause or worsen dystonic reactions, making it an inappropriate choice for treating metoclopramide-induced dystonic reactions.

**Clinical Pearl / High-Yield Fact**
When treating metoclopramide-induced dystonic reactions, it's essential to administer the anticholinergic agent promptly, as delay can lead to worsening of symptoms. A commonly used mnemonic to remember the first-line treatment is "PROMETHAZINE" – P for prompt administration, R for reducing the reaction, O for optimal outcome, M for muscle relaxation, E for effective treatment, T for timely intervention, H for halting the dystonic reaction, A for alleviating symptoms, Z for zeroing in on the correct treatment, I for immediate action, N for neutralizing the effects of metoclopramide, E for ending the dystonic reaction.

**✓ Correct Answer: B. Promethazine**