**Core Concept:** Acne drug-induced pigmentation and the role of isotretinoin, benzoyl peroxide, and hydroquinone in dermatology.

**Why the Correct Answer is Right:** The correct answer is D. Hydroquinone is a commonly used topical agent in dermatology for the treatment of hyperpigmentation disorders such as melasma, post-inflammatory hyperpigmentation, and age spots. It works by inhibiting the enzyme tyrosinase, which is responsible for melanin production. Hydroquinone competes with the substrate tyrosine, thereby reducing melanin synthesis and lightening the skin tone.

**Why Each Wrong Option is Incorrect:**

A. Isotretinoin (correct answer D) is a retinoid used for severe acne treatment and is not associated with hyperpigmentation as a side effect.
B. Benzoyl peroxide is an antimicrobicide used in acne treatment, but its mechanism does not involve hyperpigmentation.
C. Melphalan is an alkylating agent used in cancer chemotherapy and not related to acne or pigmentation.

**Why the Correct Answer is Right:** In this case, the 17-year-old girl has been taking a drug for acne treatment for 2 years, leading to pigmentation. Hydroquinone is a suitable answer as it is known to cause hyperpigmentation when used over a long duration, as seen in option D.

**Clinical Pearl:** Hydroquinone is essential for treating hyperpigmentation disorders, but its long-term use should be carefully monitored as it may lead to side effects like hyperpigmentation and hypopigmentation. This is why a well-informed dermatologist will prescribe this drug for short durations and monitor the patient closely for any adverse reactions.