**Core Concept**
The velvety appearance of the stomach is a characteristic sign of arsenic poisoning, resulting from the accumulation of arsenic in the gastric mucosa and its subsequent conversion to arsenic sulfide. This process leads to the formation of a black, velvety substance that coats the stomach lining.

**Why the Correct Answer is Right**
Arsenic poisoning causes the formation of arsenic sulfide, which precipitates out of the solution and coats the gastric mucosa, giving it a characteristic velvety appearance. This occurs due to the reaction between arsenic and sulfide ions in the stomach, resulting in the formation of a black, insoluble compound. The presence of this velvety substance is a classic sign of arsenic poisoning and can be observed during autopsy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Abrus precatorius, also known as rosary pea or crab's eye, is a plant that contains the toxin abrin. While abrin can cause gastrointestinal symptoms, it does not produce a velvety appearance of the stomach.
* **Option B:** Barbiturates are a class of sedative-hypnotic drugs that can cause a range of symptoms, including gastrointestinal disturbances. However, they do not produce a velvety appearance of the stomach.
* **Option D:** Lead poisoning can cause a range of gastrointestinal symptoms, including constipation and abdominal pain. However, it does not produce a velvety appearance of the stomach.

**Clinical Pearl / High-Yield Fact**
The velvety appearance of the stomach in arsenic poisoning is a classic sign that can be observed during autopsy. This sign is often referred to as "Arsenic's black coat" and is a key indicator of arsenic poisoning.

✓ Correct Answer: C. Arsenic