This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Disorders of the Anal Canal, Subject name: Surgery
[{'content': 'Not a Cause of acute anal pain: \nSelect and explain. \nA. Thrombosed hemorrhoids, B. Acute anal fissure, C. Fistula in ano, D. Perianal abccess', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. (c) Fistula in anoRef: Bailey & Love 26/h ed. /1259Clinical Features* Thrombosed hemorrhoids# External hemorrhoids are located distal to the dentate line and are covered with anoderm. Because the anoderm is richly innervated, thrombosis of an external hemorrhoid may cause significant pain.# Internal hemorrhoids may prolapse or bleed, but rarely become painful unless they develop thrombosis and necrosis (usually related to severe prolapse, incarceration, and/or strangulation).* Acute anal fissure# Characteristic symptoms include tearing pain with defecation and hematochezia (usually described as blood on the toilet paper)* Fistula in ano# Patients usually complain of intermittent purulent discharge (which may be bloody) and pain (which increases until temporary relief occurs when the pus discharges).# Schwartzs 9th ed and oxford textbook of surgery do not mention pain as a feature of fistula in ano* Perianal abccess# Severe anal pain is the most common presenting complaint. Walking, coughing, or straining can aggravate the pain.', 'role': 'assistant'}]