<s>[INST] context: C- Mucous membrane and circular smooth muscle layer of rectum form three transverse folds; middle one is called Houston's valve. Venous blood returns to poal venous system the superior rectal vein. A -In females, the urethra extends from bladder, runs above anterior vaginal wall, and pierces the urogenital diaphragm to reach urethral orifice in the vestibule behind the clitoris. It is about 4 cm long. In males, the urethra is about 20 cm long. B. - Obturator internus muscle and its fascia form lateral wall of ischiorectal fossa. This muscle is innervated by nerve to obturator internus, which passes through greater and lesser sciatic foramen. D. -Vagina is the genital canal in the female, extending from vestibule to uterine cervix. It transmits the products of menstruation and receives penis in copulation. E.- Ischiorectal fossa lies in anal triangle and is bound laterally by obturator internus with its fascia and superomedially by levator ani and external anal sphincter. It contains inferior rectal vessels. Thus, haemorrhage occurs in ischiorectal fossa when it is ruptured. [/INST] question: CT scan of female perineum and pelvis is given below. Which structure has Houston's valve or fold, with its venous blood drained by poal venous system? | options: A: B B: C C: A D: E | answer: B </s>