**Core Concept:** The dermatomes are territories on the skin and underlying tissues that are supplied by individual spinal nerves. Understanding dermatomal distribution is essential for understanding peripheral nerve innervation and its relation to dermatomes.

**Why the Correct Answer is Right:** The perianal skin is supplied by the S2, S3, and S4 spinal nerves. These nerves belong to the lower rectal dermatomes, which are responsible for the sensory innervation of the perianal skin.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it refers to a dermatome located on the lower limb, which is not related to the supply of perianal skin.
B. This option is wrong due to the dermatome being located on the lower limb, again unrelated to the supply of perianal skin.
C. This option is incorrect because it refers to a dermatome located on the upper limb, which is not relevant to the supply of perianal skin.
D. This option is correct as it mentions the correct dermatome (S2) responsible for the sensory innervation of the lower rectal region, including perianal skin.

**Clinical Pearl:** The dermatomes provide a useful mnemonic for remembering the distribution of sensory innervation. The "SAHM" mnemonic can be used to remember the dermatomes responsible for the sensory innervation of specific regions:

1. S - Sacral dermatomes (S2, S3, and S4) innervate the lower rectal region, including perianal skin.
2. A - Lumbar dermatomes (L1-L5) innervate the lower limb region, which is not relevant to the perianal skin supply.
3. H - Cervical dermatomes (C5-C8) innervate the upper limb region, which is irrelevant to the perianal skin supply.
4. M - Thoracic dermatomes (T1-T12) innervate the chest and thoracic region, which is not relevant to the perianal skin supply.

**Answer:** D. S2, S3, and S4 (Answer: Dermatomes S2, S3, and S4)

**Explanation:** The correct dermatomes responsible for the sensory innervation of the lower rectal region, including perianal skin, are S2, S3, and S4. These dermatomes are part of the sacral region and provide sensory innervation to the lower rectum, anus, and perianal region.

Understanding dermatomes is essential for clinical practice, as it helps in identifying the source of referred pain, dermatomal sensory deficits, and understanding the distribution of peripheral nerves. Remembering the SAHM mnemonic can help you recall the relevant dermatomes for the lower rectal region.