During childbih, which of the following muscles is most often injured by a tear of the perineum?

The fetal head is suppoed during childbih by the pelvic floor. If injury occurs during the passage of the child through the dilated uterine cervix, this injury most frequently involves a tear of the perineum between the vagina and anus. This tear usually involves the pubococcygeus, which is the main pa of the levator ani. The pubococcygeus runs from the pubic bone anteriorly to the coccyx posteriorly, and contains an anterior opening for the urethra and vagina and a posterior opening for the rectum. Tears to this muscle that heal poorly may consequently predispose for uterine prolapse, cystocele, and rectocele. The coccygeus is a relatively small muscle of the pelvic floor that runs from the ischial spine to the inferior end of the sacrum. The iliococcygeus is another pa of the levator ani, and runs from the iliac bone to the coccyx, forming pa of the more lateral aspects of the pelvic floor. The puborectalis is the third pa of the levator ani, and runs from pubic bone to pubic bone, forming a sling-like structure around the wall of the anal canal Ref: Lippincott's Illustrated Q&A Review of Anatomy and Embryology - Page 85