This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Abdomen and pelvis, Subject name: Anatomy
With anal canal rectum forms an angle of 
Select and explain. 
A. 60 degree, B. 45 degree, C. 90 degree, D. 120 degree
C: 90 degree
Answer: C: 90 degree 
Explanation: The lumen of the rectum terminates at the pelvic floor and is continued, downwards and posteriorly, as the anal canal, passing through the levator ani muscle sheet and surrounded by the internal and external anal sphincters. The anal canal is 2.5 to 5 cm in length and 3 cm in diameter when distended. The axis of the rectum forms almost a right angle (average 82 degrees) with the axis of the anal canal. It has been established by radiological studies that the anal canal is an antero-posterior slit in the resting state. Ref -pubmed.com