**Core Concept**
The Hippocratic sign, also known as the "dilated and constricted pupil sign," is a clinical indication of sympathetic nervous system overactivity. It is characterized by an alternating contraction and dilation of the pupils, which is a manifestation of the body's "fight or flight" response.

**Why the Correct Answer is Right**
In the context of poisoning, the Hippocratic sign is typically observed in cases of organophosphate or carbamate poisoning. These substances inhibit acetylcholinesterase, an enzyme responsible for the breakdown of acetylcholine, a neurotransmitter that acts on the muscarinic and nicotinic receptors in the nervous system. As a result, there is an accumulation of acetylcholine, leading to overstimulation of the muscarinic receptors in the eye, which causes the pupils to dilate. However, the nicotinic receptors in the autonomic ganglia are also overstimulated, leading to an increase in sympathetic outflow, which causes the pupils to constrict. This alternating contraction and dilation of the pupils is the hallmark of the Hippocratic sign.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Hippocratic sign is not typically associated with barbiturate poisoning, which primarily affects the central nervous system and does not directly affect the autonomic nervous system.

**Option B:** This option is incorrect because the "pinpoint pupils" sign is more commonly associated with opioid poisoning, which directly stimulates the μ-opioid receptors in the brainstem, leading to miosis.

**Option C:** This option is incorrect because the "miosis" sign is more commonly associated with anticholinergic poisoning, which inhibits the muscarinic receptors in the eye, leading to pupillary constriction.

**Option D:** This option is incorrect because the "mydriasis" sign is more commonly associated with anticholinergic poisoning, which inhibits the muscarinic receptors in the eye, leading to pupillary dilation.

**Clinical Pearl / High-Yield Fact**
The Hippocratic sign is a critical clinical finding that can help diagnose organophosphate or carbamate poisoning. It is essential to remember that this sign is not pathognomonic and can be seen in other conditions as well.

**Correct Answer:** C.