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痘坑 = Acne Scar 或 Pimple Scar
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Acne Scars - Severe acne often leaves small scars where the skin gets a "volcanic" shape. Acne scars are difficult and expensive to treat, and it is unusual for the scars to be successfully removed completely. In those cases, acne scar treatment may be appropriate....
http://health.moonlightchest.com/acne_vulgaris.asp