**Core Concept**
Syrup of ipecac is a medication used to induce vomiting in cases of poisoning. It works by stimulating the vomiting center in the brain.

**Why the Correct Answer is Right**
Syrup of ipecac contains the alkaloids emetine and cephaeline, which act on the chemoreceptor trigger zone in the medulla oblongata to induce vomiting. In the past, it was commonly used to treat poisoning in children. However, its use has been largely discontinued in many countries due to concerns about its effectiveness and potential harm. Despite this, it remains an option in some guidelines for inducing vomiting in children who have ingested a poison.

**Why Each Wrong Option is Incorrect**
**Option A:** Oral rehydration solution is used to treat dehydration and is not indicated for inducing vomiting.
**Option B:** Mustard in warm water is a folk remedy that has been used to induce vomiting, but it is not a recommended or evidence-based treatment for poisoning.
**Option C:** Apomorphine is a medication used to treat Parkinson's disease and erectile dysfunction, and it can induce vomiting as a side effect, but it is not recommended for use in children or as a treatment for poisoning.

**Clinical Pearl / High-Yield Fact**
It's essential to note that inducing vomiting is not always the best course of action in cases of poisoning, and in many cases, it can do more harm than good. The American Academy of Pediatrics recommends against using syrup of ipecac to induce vomiting and instead advises calling the national poison control center for guidance.

**✓ Correct Answer: D. Syrup of ipecac**