Welcome to Ask A Derm, a new monthly feature where board-certified dermatologist Shereene Idriss is on hand to answer your most pressing skin-related questions. Though nothing can substitute a visit to Dr. Idriss’ practice at Union Square Dermatology, think of this as your skincare primer before your appointment date arrives. Get to know Dr. Idriss a bit better through her Top Shelf here, and through her Instagram account, @shereeneidriss. Got a question you’d like Dr. Idriss to answer? Ask away at email@example.com.