A 35-year-old multiparous woman with a history of one episode of pelvic inflammatory disease (PID) 3 years ago (treated successfully with antibiotics) presents for contraceptive counseling. She has had no recurrent infections and her partner is monogamous. She requests a long-acting reversible contraceptive. On examination, her vitals are normal and pelvic examination is unremarkable. Which IUCD is most appropriate, and what is the key contraceptive mechanism?