**Core Concept**
Arlt's line is a clinical sign associated with chronic adrenal insufficiency, also known as Addison's disease. It is characterized by a white or grayish band of depigmentation on the skin, typically visible on the lower back or buttocks. This phenomenon is due to the reduced production of melanocyte-stimulating hormone (MSH) in patients with adrenal insufficiency.

**Why the Correct Answer is Right**
The reduced production of MSH, a peptide hormone produced by the pituitary gland, leads to the decreased pigmentation of skin. MSH stimulates the melanocytes to produce melanin, resulting in skin darkening. In Addison's disease, the deficiency of MSH leads to a decrease in melanin production, causing the characteristic depigmentation seen in Arlt's line. The skin in this area is most affected due to the presence of apocrine glands, which are richly innervated by the sympathetic nervous system.

**Why Each Wrong Option is Incorrect**
**Option A:** Arlt's line is not typically seen in thyroid disease, although thyroid hormone deficiency can also cause skin changes.

**Option B:** Arlt's line is not a characteristic feature of Cushing's syndrome, which is associated with an excess of cortisol and other glucocorticoids.

**Option C:** Arlt's line is not a sign of vitamin B12 deficiency, which can cause skin lesions and pigmentation changes, but these are different from Arlt's line.

**Clinical Pearl / High-Yield Fact**
Remember that Arlt's line is a specific sign of Addison's disease and is often seen in patients with chronic adrenal insufficiency. It is essential to consider this sign in patients with skin depigmentation, especially when combined with other clinical features of Addison's disease.

**Correct Answer: C. Addison's disease.**