Internal anal sphincter is pa of: (A) Internal longitudinal fibres of rectum, (B) Puborectalis muscle, (C) Deep perineal muscles, (D) Internal circular muscle fibres of rectum
Answer is D. 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