**Core Concept**
Chronic poisoning with a specific substance is characterized by a combination of neurological and systemic symptoms. The blue line in the gum is a classic sign associated with this condition, indicating exposure to a particular heavy metal.

**Why the Correct Answer is Right**
The symptoms described are consistent with chronic lead poisoning. The blue line, also known as Burton's line, is caused by the deposition of lead sulfide in the gingival margins. The tremors and personality changes are due to the neurotoxic effects of lead, which can cause damage to the nervous system, including the brain. Lead interferes with the synthesis of myelin, the fatty substance that surrounds and protects nerve fibers, leading to impaired nerve function and the characteristic symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Mercury poisoning can cause tremors and personality changes, but it is not associated with a blue line in the gum. Mercury is a neurotoxin that can damage the brain and nervous system, but the specific sign of Burton's line is not seen in mercury poisoning.

**Option B:** Arsenic poisoning can cause a range of systemic symptoms, including skin changes and neurological effects, but it is not typically associated with a blue line in the gum. Arsenic is a potent poison that can cause damage to multiple organ systems, but the specific sign of Burton's line is not characteristic of arsenic poisoning.

**Option C:** Copper poisoning can cause gastrointestinal symptoms and liver damage, but it is not typically associated with a blue line in the gum. Copper is an essential trace element, but excessive exposure can cause toxicity, including liver damage and gastrointestinal symptoms.

**Clinical Pearl / High-Yield Fact**
The blue line in the gum (Burton's line) is a classic sign of chronic lead poisoning, and its presence should raise suspicion for lead exposure in patients presenting with tremors and personality changes.

**Correct Answer:** C. Lead.