**Core Concept**
Cisplatin-induced nausea and vomiting is a form of acute chemotherapy-induced emesis, which occurs within 24 hours of chemotherapy administration. This type of emesis is mediated by the release of substance P from the terminals of the vagus nerve, leading to the activation of the chemoreceptor trigger zone (CTZ) in the area postrema.

**Why the Correct Answer is Right**
The correct answer is a 5-HT3 receptor antagonist, which is effective in preventing acute chemotherapy-induced emesis. 5-HT3 receptors are located on the terminals of the vagus nerve and in the CTZ, and their activation leads to the release of substance P and the initiation of the vomiting reflex. By blocking these receptors, 5-HT3 antagonists can prevent the activation of the CTZ and the subsequent release of substance P.

**Why Each Wrong Option is Incorrect**
**Option A:** Metoclopramide is a dopamine antagonist that can also block 5-HT3 receptors, but it is not the first-line treatment for acute chemotherapy-induced emesis.
**Option B:** Ondansetron is a 5-HT3 receptor antagonist, but it is also not the first-line treatment for acute chemotherapy-induced emesis occurring within 24 hours.
**Option C:** Dexamethasone is a corticosteroid that can be used to prevent chemotherapy-induced emesis, but it is not the primary treatment for acute emesis occurring within 24 hours.

**Clinical Pearl / High-Yield Fact**
The "5-HT3 pathway" is a key target for the prevention of acute chemotherapy-induced emesis, and 5-HT3 antagonists such as ondansetron and granisetron are effective in preventing this type of emesis.

**Correct Answer:** A. Metoclopramide