This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Abdomen and pelvis, Subject name: Anatomy
Referred pain in external hemorrhoids is due to 
Select and explain. 
A. Common pudendal nerve, B. Inferior rectal nerve, C. Splanchnic nerve, D. Sympathetic nerve
B: Inferior rectal nerve
Answer: B: Inferior rectal nerve 
Explanation: Internal hemorrhoids are located on the inside lining of the rectum and cannot be felt unless they prolapse and push through the anus opening causing pain and itching. A thrombosed external hemorrhoid occurs when blood within the blood vessel clots, and may cause significant pain and swelling.