An 18-Year-Old Woman Presents with a 2-Month History of Intermenstrual

An 18-year-old woman presents with a 2-month history of intermenstrual bleeding, increased vaginal discharge, and mild lower abdominal pain. Her last menstrual period (LMP) was 1 week ago, and her cycles are regular. Sexual debut was 1 year ago; she is sexually active with the same partner (her only partner) and has taken oral contraception pills for 1 year. She reports taking her oral contraceptives consistently with no side effects. The most likely cause of this bleeding is:

A) Chlamydia infection
B) Endometriosis
C) Cervical polyps
D) Missing oral contraceptive (OC) pills