**Question:** Which of the following does not contribute to the formation of anorectal ring in human beings?

A. Sphincter muscles
B. Anal cushions
C. Pelvic floor muscles
D. Perianal skin

**Core Concept:** The anorectal ring is a critical structure in the anal canal, consisting of a combination of various components that work together to facilitate defecation and maintain continence. These components include sphincter muscles, anal cushions, pelvic floor muscles, and perianal skin.

**Why the Correct Answer is Right:** Option D, perianal skin, is not directly involved in the formation of the anorectal ring. The anorectal ring primarily comprises of sphincter muscles, anal cushions, and pelvic floor muscles, all working together to maintain continence and facilitate defecation. Perianal skin is an essential part of the skin folds around the anus, but it does not contribute to the formation of the anorectal ring itself.

**Why Each Wrong Option is Incorrect:**

A. Sphincter muscles: The internal and external anal sphincter muscles are crucial components of the anorectal ring, as they help maintain continence and control the passage of stool.

B. Anal cushions: Anal cushions, also known as Haemorrhoidal cushions, are part of the anal canal and contribute significantly to the formation of the anorectal ring.

C. Pelvic floor muscles: The pelvic floor muscles also play a vital role in the formation of the anorectal ring, working alongside the sphincter muscles and anal cushions to facilitate defecation and maintain continence.

**Clinical Pearl:** The formation of the anorectal ring involves a complex interaction between various structures, including sphincter muscles, anal cushions, and pelvic floor muscles. While perianal skin is essential for the overall integrity of the anal canal, it does not directly contribute to the formation of the anorectal ring. Understanding the anatomy and function of these structures is crucial for understanding defecation and rectal motility disorders.