**Core Concept**
Riehl's melanosis is a rare, acquired, pigmentation disorder characterized by the appearance of bluish-black or dark blue patches on the skin. It is an immune-mediated condition that involves a hypersensitivity reaction to certain allergens, often resulting in the formation of melanin.

**Why the Correct Answer is Right**
Riehl's melanosis is thought to occur due to an allergic reaction to certain substances, such as fragrances, dyes, or other chemicals, which triggers an immune response. This response leads to the activation of melanocytes, resulting in the production of excess melanin and the characteristic pigmentation of the skin. The exact mechanism is not fully understood, but it is believed to involve the release of histamine and other chemical mediators.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Riehl's melanosis is not caused by a genetic mutation, but rather by an allergic reaction to external substances.
**Option B:** This option is incorrect because Riehl's melanosis is not characterized by the formation of blisters or pustules, but rather by the appearance of pigmented patches on the skin.
**Option C:** This option is incorrect because Riehl's melanosis is not typically associated with systemic symptoms such as fever or joint pain, but rather with localized skin lesions.

**Clinical Pearl / High-Yield Fact**
Riehl's melanosis is often confused with other pigmentation disorders, such as fixed drug eruption or lichen planus, but it has a distinctive clinical appearance and is typically associated with a history of exposure to allergens.

**Correct Answer:** A.