<prompt> Treatment of choice in severe cystic acne:IsotretinoinTretinoinBenzoyl peroxideAzelaic acid  <response>: Ans. a IsotretinoinRef Katzung 11th ed. 1308, 1455; KDT 854 13-cis-Retinoic acid Isotretinoin is an analogue of vitamin A. It is used for the treatment of severe cystic acne Isotretinoin Accutane is a synthetic retinoid currently restricted to the treatment of severe cystic acne that is refractory to standard therapies. It appears to act by inhibiting sebaceous gland size and function. Half life-10-20 hours. Common side effect: hypervitaminosis A, dryness of skin and mucous membrane. Teratogenic effect: Extremely high risk of CNS, face, ear, and other malformations. Risk is equal throughout all the three trimester.Also Know AI-trans vitamin A Tretinoin: effective topical agent in the treatment of acne vulgaris. It is a potent comedolytic and stimulate epidermal cell turnover, resulting in peeling. Benzoyl peroxide is an effective topical agent in the treatment of acne vulgaris MOA of benzoyl peroxide- its antimicrobial activity against P acnes and to its peeling and comedolytic effects. Azelaic acid effective in the treatment of acne vulgaris. MOA of azelaic acid- antimicrobial activity against P acnes as well as an in vitro inhibitory effect on the conversion of testosterone to dihydrotestosterone. 