<prompt> Internal anal sphincter is pa ofInternal longitudinal fibres of rectumPuborectalis muscleDeep perineal musclesInternal circular muscle fibres of rectum  <response>: 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 