Which agent is best for treating a 17-year-old female's acne with the goal of lightening darkened spots on her face?

Azelaic acid is an effective choice for treating acne, especially in patients seeking to address post-inflammatory hyperpigmentation, or darkened spots on the skin. This agent uniquely combines anti-inflammatory properties with the ability to inhibit the activity of certain enzymes involved in the pathways leading to the formation of acne. Moreover, azelaic acid has been shown to promote skin cell turnover, which helps in lightening dark spots and improving skin texture.

While benzoyl peroxide is excellent for managing acne by killing bacteria and reducing oil production, it does not specifically target hyperpigmentation. Clindamycin is an antibiotic used to reduce acne-related inflammation but does not have significant effects on hyperpigmented areas. Dapsone, while used in some acne treatments, also does not primarily address pigmentation issues.

Thus, azelaic acid not only treats active acne lesions but is also the best option for lightening dark marks, making it a suitable choice for the patient described.