**Core Concept**
Aldrich-Mee's lines refer to a specific pattern of nail changes associated with heavy metal poisoning, characterized by white bands or transverse grooves on the nails. This phenomenon is a result of the disruption of keratinization caused by a particular heavy metal, leading to alterations in nail growth and appearance.

**Why the Correct Answer is Right**
Aldrich-Mee's lines are characteristic of arsenic poisoning. Arsenic exposure can lead to changes in the nail bed, resulting in the formation of white bands or grooves. This is due to the inhibition of keratinization by arsenic, which disrupts the normal process of nail growth. The arsenic binds to the keratin protein, causing it to become more fragile and prone to breaking, leading to the characteristic Aldrich-Mee's lines.

**Why Each Wrong Option is Incorrect**
**Option A:** Mercury poisoning typically presents with Mees' lines, which are similar but distinct from Aldrich-Mee's lines. Mees' lines are characterized by a more diffuse and scattered pattern of nail changes.
**Option B:** Lead poisoning does not typically present with Aldrich-Mee's lines. Lead poisoning can cause a range of symptoms, including abdominal pain, constipation, and neurological effects, but it is not commonly associated with this specific nail pattern.
**Option C:** Thallium poisoning can cause hair loss and nail changes, but it is not typically associated with Aldrich-Mee's lines. Thallium poisoning can cause a range of systemic effects, including neurological and gastrointestinal symptoms.

**Clinical Pearl / High-Yield Fact**
Aldrich-Mee's lines are a specific and characteristic finding in arsenic poisoning, and should be considered in patients presenting with this pattern of nail changes. This can be a valuable diagnostic clue in cases where arsenic exposure is suspected.

**Correct Answer:** A. Arsenic.