uk-drcog MCQ (SBA): Pink Discharge - Cervical Ectropion in Pregnancy

A 29-year-old woman at 26 weeks of gestation presents to the GP with a 3-day history of painless vaginal discharge tinged pink. She is group B streptococcus unknown and has had no recent cervical screening. Speculum examination shows a closed os with mucoid discharge and no active bleeding. Fetal movements are normal. Her placenta was normally sited at 20 weeks. Which is the single most likely cause?