**Core Concept**
Dextromethorphan is a non-opioid cough suppressant used in over-the-counter medications, while codeine is an opioid analgesic with antitussive properties. The distinction between these two compounds is crucial in understanding their mechanisms of action, side effects, and potential for abuse.

**Why the Correct Answer is Right**
Dextromethorphan acts centrally, primarily on the brainstem, to suppress coughing. Unlike codeine, dextromethorphan does not stimulate the mu-opioid receptors responsible for analgesia and constipation. Instead, dextromethorphan inhibits the cough reflex by increasing the threshold for coughing. This action is distinct from codeine, which also activates kappa-opioid receptors, contributing to its antitussive effects and gastrointestinal side effects, including constipation.

**Why Each Wrong Option is Incorrect**
**Option A:** Naloxone is an opioid antagonist that reverses the effects of opioids, including codeine, but does not affect dextromethorphan's antitussive action. Dextromethorphan's mechanism of action is distinct from opioid receptors.
**Option B:** Both codeine and dextromethorphan can depress mucociliary function, although the exact mechanisms and clinical significance of this effect are not well understood. This option is incorrect because it does not specifically distinguish between the two compounds.
**Option C:** Addiction is indeed a concern with codeine, particularly with prolonged use or in individuals with a history of substance abuse. However, dextromethorphan has a lower potential for abuse and addiction compared to codeine.

**Clinical Pearl / High-Yield Fact**
When prescribing cough medications, it's essential to consider the patient's history of opioid use or potential for addiction. Dextromethorphan is often preferred in patients with a history of opioid abuse or in those at risk of addiction.

**✓ Correct Answer: D. Causes no constipation**