This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pelvis and Perineum 3, Subject name: Anatomy
[{'content': 'Internal anal sphincter is a pa of \nSelect and explain. \nA. Internal longitudinal fibers of rectum, B. Internal circular muscle fibers of rectum, C. Puborectalis muscle, D. Deep perineal muscles', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Internal anal sphincter is formed by thickened circular muscle coat of the anal canal (which is continuous with rectum) and surrounds the upper two-third of the canal.', 'role': 'assistant'}]