This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'The following are important in the maintenance of normal fecal continence except - \nSelect and explain. \nA. Anorectal angulation, B. Rectal innervation, C. Internal sphincter, D. Haustral valve', 'role': 'user'}, {'content': "Answer: D \nExplanation: Normal fecal continence requires\n\nadequate rectal wall compliance to accommodate the fecal bolus.\nappropriate neurogenic control of the pelvic floor and sphincter mechanism, and\nfunctional internal and external sphincter muscles\n\nAt rest the puborectalis muscle creates a 'sling' around the distal rectum, forming a relatively acute angle (Anorectal angle). Anorectal angle distributes intraabdominal forces onto the pelvic floor and play an important role in the continence mechanism. At the time of defecation, this angle straightens, allowing the downward force to be applied along the axis of the rectum and anal canal.\nThe internal & external sphincters are tonically active at rest.\n\nthe internal sphincter is responsible for most of the resting, involuntary sphincter force\nthe external sphincter is responsible for voluntary sphincter force.\nBoth the internal and external sphincters are innervated by pudendal nerve.\nFinally, the hemorrhoidal cushions may contribute to continence by mechanically blocking the anal canal.\nThus impaired continence may result from :\npoor rectal compliance\ninjury to internal and/or external sphincter or puborectalis, or\nnerve damage or neuropathy", 'role': 'assistant'}]