**Core Concept**
The question is testing knowledge of antiemetic medications, specifically those used to counteract chemotherapy-induced nausea and vomiting (CINV). The underlying principle involves the use of antagonists to block the action of peptides involved in the emetic response.

**Why the Correct Answer is Right**
Aprepitant is a selective and competitive antagonist of the neurokinin 1 (NK1) receptor. By blocking this receptor, aprepitant prevents the release of substance P, a peptide that plays a crucial role in the emetic response. The NK1 receptor is located in the area postrema, a region of the brain involved in the induction of vomiting. By inhibiting this receptor, aprepitant reduces the frequency and severity of CINV.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ondansetron is a 5-HT3 receptor antagonist, not a peptide antagonist. While it is used to prevent CINV, it works through a different mechanism.
* **Option B:** Metoclopramide is a dopamine antagonist, primarily used to treat nausea and vomiting in the gastrointestinal tract. It is not a peptide antagonist and is not specifically used to reduce CINV.
* **Option C:** Dexamethasone is a corticosteroid used to reduce inflammation and is sometimes used as an antiemetic, but it is not a peptide antagonist.

**Clinical Pearl / High-Yield Fact**
Aprepitant is often used in combination with other antiemetics, such as ondansetron and dexamethasone, to provide comprehensive protection against CINV. This combination is known as a "3+1" regimen.

**Correct Answer:** C. Aprepitant