**Core Concept**
Constipation is a gastrointestinal symptom characterized by infrequent bowel movements, hard stools, and difficulty passing stools. It can be caused by various factors, including medications, which can slow down gut motility, increase water absorption, or alter bowel habits.

**Why the Correct Answer is Right**
The correct answer is a medication that does not cause constipation. Most medications that cause constipation act by slowing down gut motility, increasing water absorption, or altering bowel habits. For example, opioids like morphine inhibit the release of acetylcholine, a neurotransmitter that stimulates gut motility, leading to constipation. Similarly, anticholinergic medications like atropine block the action of acetylcholine, leading to decreased gut motility and constipation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Opioids like morphine are well-known to cause constipation by inhibiting the release of acetylcholine and decreasing gut motility.
* **Option B:** Anticholinergic medications like atropine block the action of acetylcholine, leading to decreased gut motility and constipation.
* **Option C:** Calcium channel blockers like verapamil can cause constipation by relaxing smooth muscle in the gut and reducing gut motility.

**Clinical Pearl / High-Yield Fact**
It's essential to note that constipation is a common side effect of many medications, and patients should be counseled about the potential risks and benefits of their medications. A thorough medication history is crucial in diagnosing and managing constipation.

**Correct Answer:** D