This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Anatomy
Internal anal sphincter is a apa of: 
Select and explain. 
A. Puborectalis muscle, B. Deep perineal muscles, C. Internal longitudinal fibers, D. Internal circular fibers
D: Internal circular fibers
Answer: D: Internal circular fibers 
Explanation: Involuantry internal anal sphincter is formed by thickening of circular muscle layer (i.e. circular layer of muscularis externa, a smooth muscle) of upper end (2/3 or 3/4) of anal canal. This sphincter remains in the state of tonic contraction most of the time to maintain resting tone or pressures (-90 cm H20) & to prevent leakage of fluid or flatus. Its contraction (tonus) is maintained by sympathetic fibers from superior rectal (periaerial) and hypogastric plexuses; and inhibited (i.e. sphincter relaxed) by parasympathetic pelvic NERVES. Ref: Gray&;s anatomy text book of anatomy BD chaurasia 21st Ed.