Ans: a (Acne) Ref: Tripathi, 6th ed, p. 852; Katzung, 10th ed, p. 1000BENZOYL PEROXIDEBenzoyl peroxide is an effective topical agent in the treatment of acne vulgaris. It penetrates the stratum comeum or follicular openings unchanged and is converted metabolically to benzoic acid within the epidermis and dermis.RETINOIC ACID & DERIVATIVESRetinoic acid, also known as tretinoin or all-trans-retmoic acid, is the acid form of vitamin A. It is an effective topical treatment for acne vulgaris. The most common adverse effects of topical retinoic acid are erythema and dryness that occur in the first few weeks of useAdapalene is a derivative of naphthoic acid that resembles retinoic acid in structure and effects. It is applied as a 0.1 % gel once daily.. Adapalene is less irritating than tretinoin and is most effective in patients with mild to moderate acne vulgaris.Tazarotene is an acetylenic retinoid that is available as a 0.1 % gel and cream for the treatment of mild to moderately severe facial acne. Topical tazarotene should be used by women of childbearing age only after contraceptive counseling. It is recommended that tazarotene should not be used by pregnant women.ISOTRETINOINIsotretinoin is a synthetic retinoid currently restricted to the treatment of severe cystic acne that is recalcitrant to standard therapies. Most patients with cystic acne respond to 1-2 mg/kg, given orally in two divided doses daily for 4-5 months Common adverse effects resemble hypervitaminosis A and include dryness and itching of the skin and mucous membranes. Teratogenicity is a significant risk in patients taking isotretinoin.AZELAICACIDAzelaic acid is a straight-chain saturated dicarboxylic acid that has been demonstrated to be effective in the treatment of acne vulgaris and acne rosacea.