**Core Concept**
Mees lines, also known as Mee's lines or leukonychia striata, are transverse white bands that appear on the nails due to a disturbance in the nail matrix. This condition is typically associated with a systemic illness or exposure to certain toxins.

**Why the Correct Answer is Right**
Arsenic poisoning is characterized by the appearance of Mees lines on the nails, which are a result of the disruption of the nail matrix. Arsenic exposure leads to the destruction of the nail matrix, causing the formation of white bands or lines on the nails. This is due to the arsenic-induced damage to the nail matrix cells, which are responsible for the production of the nail plate. The Mees lines in arsenic poisoning typically appear 2-3 weeks after exposure and can persist for several months.

**Why Each Wrong Option is Incorrect**
**Option A:** Phosphorus poisoning does not typically cause Mees lines. While phosphorus exposure can lead to various systemic effects, Mees lines are not a characteristic manifestation.
**Option C:** Lead poisoning can cause various neurological and hematological effects, but Mees lines are not a specific or characteristic feature of lead poisoning.
**Option D:** Thallium poisoning can cause hair loss, skin lesions, and gastrointestinal symptoms, but Mees lines are not typically associated with thallium exposure.

**Clinical Pearl / High-Yield Fact**
Mees lines can also be seen in other conditions such as zinc deficiency, mercury poisoning, and in patients with chronic kidney disease. However, arsenic poisoning is the most well-known and classic cause of Mees lines.

**✓ Correct Answer: B. Arsenic**