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
Internal anal sphincter is pa of 
Select and explain. 
A. Internal longitudinal fibres of rectum, B. Puborectalis muscle, C. Deep perineal muscles, D. Internal circular muscle fibres of rectum
D: Internal circular muscle fibres of rectum
Answer: D: Internal circular muscle fibres of rectum 
Explanation: The internal anal sphincter, IAS, (or sphincter ani internus) is a muscular ring that surrounds about 2.5-4.0 cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter. It is about 5 mm thick and is formed by an aggregation of the involuntary circular fibers of the rectum. Its lower border is about 6 mm from the orifice of the anus. Ref - BDC 6e vol2 pg413-416