**Question:** All are antiemetics except -

A. Ondansetron
B. Domperidone
C. Dexamethasone
D. Metoclopramide

**Core Concept:** Antiemetics are medications that help prevent or reduce nausea and vomiting. They are commonly used in clinical practice to manage side effects of chemotherapy or radiation therapy, as well as postoperative nausea and vomiting.

**Why the Correct Answer is Right:** Dexamethasone is a glucocorticoid hormone with anti-inflammatory and immunosuppressive properties. Although it is often used in clinical practice to control nausea and vomiting, it is not an antiemetic in the strict sense. Antiemetics specifically target the mechanisms involved in emesis (vomiting) and are more focused on the central nervous system (CNS) modulation.

**Why Each Wrong Option is Incorrect:**

A. Ondansetron is a selective serotonin receptor antagonist, specifically targeting 5-HT3 receptors in the gut and brain. It is commonly used in the management of chemotherapy-induced nausea and vomiting.

B. Domperidone is a dopamine D2 receptor antagonist, primarily acting on the gastrointestinal tract to reduce nausea and vomiting.

C. Dexamethasone, as mentioned earlier, is a glucocorticoid hormone with anti-inflammatory and immunosuppressive effects, but not specifically designed for antiemetic purposes.

D. Metoclopramide is a dopamine D2 receptor antagonist, similar to domperidone, and is often used in the treatment of nausea and vomiting, especially in post-operative patients.

**Clinical Pearl:** The correct use of antiemetics based on their mechanisms of action is essential to effectively manage nausea and vomiting in patients. Choosing the appropriate antiemetic based on the type of emesis (e.g., chemotherapy-induced, post-operative, etc.) ensures optimal patient care and minimizes side effects associated with different medications.