## Core Concept
The question pertains to the diagnosis of metal poisoning based on specific nail changes. Certain heavy metals can cause distinct alterations in nail pigmentation, texture, and morphology due to their toxic effects on the nail matrix and bed.

## Why the Correct Answer is Right
The correct answer, , corresponds to Arsenic poisoning. Arsenic exposure can lead to characteristic nail changes, including:
- **Mees' lines**: White lines that appear across the nails due to arsenic's toxic effect on the nail matrix.
- **Thickening and brittleness**: Arsenic can cause nails to become thickened and brittle.
- **Hyperpigmentation and depigmentation**: Nails may exhibit areas of increased or decreased pigmentation.

## Why Each Wrong Option is Incorrect
- **Option A:** . Lead poisoning primarily affects the hematopoietic, nervous, gastrointestinal, and renal systems. While it can cause various symptoms, characteristic nail changes like those associated with arsenic poisoning are not typically seen.
- **Option B:** . Mercury poisoning mainly affects the nervous, digestive, and immune systems. It does not characteristically cause specific nail changes like Mees' lines.
- **Option D:** . Copper poisoning can cause gastrointestinal symptoms and, in severe cases, liver and kidney damage. However, it is not commonly associated with the specific nail changes described.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that Mees' lines can be caused by several heavy metals, including arsenic, thallium, and mercury, as well as by other conditions such as high fever, chemotherapy, and certain infections. However, the combination of nail changes, including Mees' lines, thickening, and pigmentation changes, is particularly suggestive of arsenic poisoning.

## Correct Answer Line
**Correct Answer: B. Arsenic.**