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
Internal anal sphincter is a pa of 
Select and explain. 
A. Internal longitudinal fibers of rectum, B. Internal circular muscle fibers of rectum, C. Puborectalis muscle, D. Deep perineal muscles
B: Internal circular muscle fibers of rectum
Answer: B: Internal circular muscle fibers of rectum 
Explanation: 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.