**Core Concept**
Cisplatin-induced vomiting is a common side effect of chemotherapy, particularly on the third day of treatment. It is caused by the release of serotonin from the enterochromaffin cells in the small intestine, which stimulates the vomiting center in the brain.

**Why the Correct Answer is Right**
The correct answer is related to the use of 5-HT3 antagonists, which are effective in preventing cisplatin-induced vomiting. These agents block the action of serotonin at the 5-HT3 receptors on the vagus nerve, thereby reducing the stimulus to the vomiting center. Metoclopramide, a dopamine antagonist, also has antiemetic properties and can be used in combination with 5-HT3 antagonists.

**Why Each Wrong Option is Incorrect**
**Option A:** Ondansetron is a 5-HT3 antagonist and would be an effective treatment for cisplatin-induced vomiting.
**Option B:** Metoclopramide is a dopamine antagonist and can be used to prevent chemotherapy-induced nausea and vomiting, but it is not the best choice for cisplatin-induced vomiting on the third day of treatment.
**Option C:** Dexamethasone is a corticosteroid that can be used to prevent chemotherapy-induced nausea and vomiting, but it is not the primary treatment for cisplatin-induced vomiting.
**Option D:** Prochlorperazine is a dopamine antagonist that can be used to treat nausea and vomiting, but it is not the best choice for cisplatin-induced vomiting.

**Clinical Pearl / High-Yield Fact**
The timing of cisplatin-induced vomiting is often related to the release of serotonin from the enterochromaffin cells, which can be blocked by 5-HT3 antagonists.

**Correct Answer: B. Metoclopramide.**