Small amount of atropine is added to diphenoxylate because
**Core Concept**
The underlying principle being tested is the concept of **pharmacological manipulation** to prevent abuse of certain drugs. Diphenoxylate is an **opioid** used to treat diarrhea, and atropine is an **anticholinergic** agent.
**Why the Correct Answer is Right**
Atropine is added to diphenoxylate to discourage **recreational use** due to its unpleasant side effects at higher doses. When taken in excess, the atropine component causes undesirable effects such as **tachycardia**, **dryness of mouth**, and **dilated pupils**.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because atropine does not enhance the therapeutic effect of diphenoxylate.
**Option B:** Incorrect as atropine's primary role here is not to reduce side effects but to deter misuse.
**Option C:** Incorrect because atropine is not added to improve the bioavailability of diphenoxylate.
**Option D:** Incorrect as the primary purpose of adding atropine is not to treat a specific condition but to prevent abuse.
**Clinical Pearl / High-Yield Fact**
Remember, the combination of diphenoxylate and atropine is a classic example of how **abuse-deterrent formulations** can be designed to prevent the misuse of **controlled substances**.
**Correct Answer:** D. to discourage abuse.