**Core Concept**
Riehl's melanosis is a rare skin condition characterized by an abnormal pigmentation reaction, typically seen in the context of allergic contact dermatitis. It is a type of exogenous ochronosis, which occurs due to the accumulation of homogentisic acid in the skin, leading to the deposition of a bluish-black pigment.

**Why the Correct Answer is Right**
The condition arises from the oxidation of hydroquinone, a common ingredient in hair dyes, skin-lightening creams, and other cosmetics. The oxidation of hydroquinone leads to the formation of a reactive intermediate, which then reacts with collagen to form homogentisic acid. This results in the accumulation of the pigment in the dermal-epidermal junction, causing the characteristic blue-black discoloration.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as Riehl's melanosis is not primarily associated with the use of coal tar or other polycyclic aromatic hydrocarbons.
* **Option B:** This option is incorrect as fixed drug eruption is a different condition that presents with well-demarcated, recurrent lesions, often in response to a specific medication.
* **Option C:** This option is incorrect as erythema nodosum is a distinct condition characterized by painful, tender nodules on the lower extremities, often associated with infections or autoimmune disorders.

**Clinical Pearl / High-Yield Fact**
Riehl's melanosis is often seen in individuals who use hair dyes or skin-lightening creams containing hydroquinone, and it can be a diagnostic clue for occupational exposure to these chemicals.

**Correct Answer: D. Hydroquinone**