**Core Concept**
Opioids exert central nervous system effects primarily through μ, κ, and δ opioid receptors, leading to analgesia, sedation, and effects on autonomic and motor functions. Key actions include respiratory depression, sedation, miosis, and suppression of reflexes like cough.

**Why the Correct Answer is Right**
Opioids cause **miosis** (constriction of the pupils) via stimulation of the parasympathetic system in the Edinger-Westphal nucleus. **Cough suppression** occurs due to inhibition of the cough reflex in the medulla. **Truncal rigidity** (increased muscle tone) is a feature of opioid-induced central nervous system depression, especially in severe cases. However, **anti-emesis** is not a primary or consistent pharmacological action of opioids. While some opioids may have mild antiemetic effects, this is not a hallmark or reliable action, unlike the others.

**Why Each Wrong Option is Incorrect**
Option A: Cough suppression is a well-documented effect of opioids due to inhibition of the cough center in the medulla.
Option C: Miosis is a classic sign of opioid use, mediated by activation of parasympathetic pathways via opioid receptors.
Option D: Truncal rigidity is observed in opioid-induced central depression, especially in severe cases or with overdose.

**Clinical Pearl / High-Yield Fact**
Miosis, respiratory depression, and sedation are hallmark signs of opioid toxicity — remember the "M" in **MOR** (Mu Opioid Receptor) and the classic triad: **miosis, respiratory depression, and sedation**. Anti-emesis is not a reliable or consistent action of opioids and should not be listed as a primary pharmacological effect.

✓ Correct Answer: B. Anti-emesis