**Core Concept**
The 5-HT3 receptor is a subtype of serotonin receptor that plays a crucial role in the chemoreceptor trigger zone (CTZ) of the brain, mediating nausea and vomiting. This receptor is specifically targeted by antiemetic medications to prevent chemotherapy-induced nausea and vomiting (CINV).

**Why the Correct Answer is Right**
The 5-HT3 receptor is activated by serotonin, which is released from the enterochromaffin cells of the small intestine in response to chemotherapy. This leads to the stimulation of the CTZ, resulting in the activation of the vomiting center. Selective 5-HT3 receptor antagonists, such as ondansetron and granisetron, are effective in preventing CINV by blocking the action of serotonin at the 5-HT3 receptor. This is why these medications are commonly used as first-line treatment for CINV.

**Why Each Wrong Option is Incorrect**
**Option A:** Histamine H1 receptor is involved in allergic reactions and is targeted by antihistamines, but it is not the primary receptor involved in chemotherapy-induced vomiting.

**Option C:** Dopamine D2 receptor is involved in the regulation of movement and is targeted by antipsychotic medications, but it is not directly involved in the pathophysiology of CINV.

**Option D:** Opioid µ receptor is involved in pain modulation and is targeted by opioid analgesics, but it is not the primary receptor involved in chemotherapy-induced vomiting.

**Clinical Pearl / High-Yield Fact**
The 5-HT3 receptor is a key target for antiemetic medications, and selective antagonists are effective in preventing CINV. This is why ondansetron and granisetron are commonly used as first-line treatment for CINV.

**✓ Correct Answer: B. Serotonin 5–HT3 receptor**