This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Which of the following about the valves of Houston is true? 
Select and explain. 
A. They disappear after mobilization of the rectum, B. The middle valve folds towards the right side, C. The upper valve corresponds to the anterior peritoneal reflection, D. The valves contain all layers of the muscle wall
A: They disappear after mobilization of the rectum
Answer: A: They disappear after mobilization of the rectum 
Explanation: A i.e. They disappear after mobilization of rectum