**Core Concept**
Naloxone and other opioid antagonists are used to reverse opioid-induced respiratory depression, but they also have distinct effects on gastrointestinal motility compared to opioids themselves. Opioids typically decrease gastric motility by acting on opioid receptors in the gastrointestinal tract.

**Why the Correct Answer is Right**
Naloxone acts as an opioid antagonist by competitively binding to opioid receptors, thereby reversing the effects of opioids. Unlike opioids, naloxone does not stimulate the mu-opioid receptors in the gastrointestinal tract, which are responsible for decreasing gastric motility. Instead, naloxone blocks these receptors, thereby allowing normal gastric motility to resume. This is why naloxone is used to treat opioid-induced constipation and other gastrointestinal side effects.

**Why Each Wrong Option is Incorrect**
* **Option B:** Morphine is an opioid agonist that stimulates mu-opioid receptors in the gastrointestinal tract, leading to decreased gastric motility. Morphine is commonly used for pain relief but can cause constipation as a side effect.
* **Option C:** Codeine is another opioid agonist that decreases gastric motility by stimulating mu-opioid receptors in the gastrointestinal tract. Codeine is often used for cough suppression and pain relief.
* **Option D:** Pethidine (also known as meperidine) is an opioid agonist that decreases gastric motility by stimulating mu-opioid receptors in the gastrointestinal tract. Pethidine is used for pain relief but has a shorter duration of action compared to other opioids.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that opioids can cause constipation by decreasing gastric motility, which can lead to bowel obstruction in severe cases. Naloxone can be used to reverse this effect, but it's crucial to note that naloxone has a shorter duration of action compared to opioids, and repeated doses may be required.

**✓ Correct Answer: A. Naloxone. Naloxone decreases gastric motility except among opioid antagonists.**