Which dermatome is on the medial aspect of each buttock (anal wink)?

• A. L3
• B. S3
• C. C6
• D. S1

Answer: (B)

Skin areas map to specific spinal nerve roots, and the gluteal region is supplied by the sacral dermatomes. The medial aspect of the buttock aligns with the S3 dermatomal distribution. The anal wink is a sacral reflex involving the S2–S4 segments (pudendal nerve pathway): touching the perianal skin can cause contraction of the external anal sphincter, and this reflex tests the integrity of those sacral segments. Since the medial buttock region corresponds to S3, that area is the one associated with the sensory input for this reflex. Other regions like the anterior thigh (L3), the arm (C6), or the lateral/posterior thigh and leg (S1) do not map to this medial buttock area, so they’re not the correct dermatomal match.