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
Dermatomal supply of the perianal skin is 
Select and explain. 
A. S1, B. L2, C. L3, D. S4
D: S4
Answer: D: S4 
Explanation: The Inferior rectal nerves (inferior anal nerves, inferior hemorrhoidal nerve) usually branch from the pudental nerve but occasionally arises directly from the sacral plexus; they cross the ischiorectal fossa along with the inferior hemorrhoidal vessels, toward the anal canal and the lower end of the rectum, and is distributed to the spincher ani externa and to the integument (skin) around the anus. image ref - Netter atlas