**Core Concept**
Erethism refers to a set of symptoms including emotional instability, irritability, and personality changes, often associated with chronic exposure to certain heavy metals. This condition is a classic example of a toxic effect on the nervous system.

**Why the Correct Answer is Right**
Mercury (Hg) is a well-known neurotoxin that can cause erethism due to its ability to disrupt the normal functioning of the central nervous system. The exact mechanism involves the inhibition of acetylcholinesterase, an enzyme crucial for neurotransmitter regulation. As a result, excessive accumulation of acetylcholine can lead to overstimulation of neurons, resulting in symptoms such as tremors, mood swings, and personality changes.

**Why Each Wrong Option is Incorrect**
**Option B:** Arsenic (As) is primarily associated with gastrointestinal and cardiovascular effects, such as nausea, vomiting, and cardiovascular collapse, rather than erethism.
**Option C:** Lead (Pb) exposure is more commonly linked to hematological, gastrointestinal, and renal effects, including anemia, abdominal pain, and kidney damage.
**Option D:** Copper (Cu) toxicity can lead to hepatic and gastrointestinal symptoms, but it is not typically associated with the characteristic symptoms of erethism.

**Clinical Pearl / High-Yield Fact**
Heavy metal poisoning can manifest with a wide range of symptoms, and a thorough medical history, including occupational exposure, is essential for accurate diagnosis.

**✓ Correct Answer: A. Hg**