Final answer:
Pelvic inflammatory disease (PID) is the most common cause of tubal factor infertility. It is an infection that can lead to scarring and blockage of the fallopian tubes.
Step-by-step explanation:
The most common cause of tubal factor infertility is Pelvic inflammatory disease (PID). PID is an infection of the female reproductive organs, specifically the uterus, cervix, fallopian tubes, and ovaries. It is often caused by sexually transmitted bacterial infections like Neisseria gonorrhoeae and Chlamydia trachomatis. The infection can lead to scarring and blockage of the fallopian tubes, preventing fertilization of the eggs.