**Core Concept**
Mee's line is a transverse white or yellowish discoloration of the fingernails or toenails, indicative of a systemic condition affecting the nail matrix. This phenomenon is associated with an underlying pathological process affecting the nail plate's growth.

**Why the Correct Answer is Right**
Mee's line is a classic sign of nitrous acid poisoning, which causes damage to the nail matrix, resulting in a white or yellowish discoloration of the nail plate. The mechanism behind Mee's line involves the disruption of the nail matrix's ability to produce healthy nail cells, leading to the characteristic appearance of the nail. Nitrous acid poisoning can occur due to the ingestion of nitric acid or its fumes, which can cause severe damage to the skin, mucous membranes, and internal organs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Mee's line is not a diagnostic feature of arsenic poisoning, although arsenic can cause changes in the nails, such as white lines or pigmentation changes, these are different from Mee's line.
**Option B:** This option is incorrect as Mee's line is not a diagnostic feature of zinc deficiency, which can cause changes in the nails, such as brittle nails or nail loss, but not Mee's line.
**Option C:** This option is incorrect as Mee's line is not a diagnostic feature of syphilis, although syphilis can cause changes in the nails, such as Hutchinson's sign or snail track nails, these are different from Mee's line.

**Clinical Pearl / High-Yield Fact**
Mee's line is a classic example of a systemic condition affecting the nail matrix, and its presence should prompt a thorough investigation into the patient's exposure history, particularly to nitrous acid.

**Correct Answer: D. Nitrous acid poisoning.**