**Core Concept**
Constipation is a gastrointestinal symptom characterized by infrequent bowel movements, hard stools, or difficulty passing stools. Various medications can contribute to constipation by altering bowel transit time, stool consistency, or gut motility.

**Why the Correct Answer is Right**
Constipation is often caused by medications that decrease gut motility, such as anticholinergics (e.g., antihistamines, antipsychotics, and antidepressants) and opioids. These medications act by blocking the action of acetylcholine at muscarinic receptors in the enteric nervous system, leading to decreased gut contractions and slowed bowel transit time.

**Why Each Wrong Option is Incorrect**
**Option A:** Opioids are a well-known cause of constipation due to their action on mu receptors in the gut, which decreases gut motility and increases water absorption.
**Option B:** Anticholinergics, such as atropine and scopolamine, also contribute to constipation by blocking muscarinic receptors in the enteric nervous system, leading to decreased gut contractions and slowed bowel transit time.
**Option C:** Calcium channel blockers (e.g., verapamil) can cause constipation due to their effects on smooth muscle contraction and relaxation in the gut.

**Clinical Pearl / High-Yield Fact**
When assessing a patient with constipation, it's essential to consider their medication regimen, including both prescription and over-the-counter medications, as many common medications can contribute to this symptom.

**Correct Answer:** D