Identify a muscle responsible for exit of waste material from the body.

The correct answer is Sphincter.

Let us understand why the Sphincter muscle is responsible for the exit of waste material from the body by analyzing each option carefully.

What is a Sphincter Muscle?
A sphincter is a specialized circular (ring-shaped) muscle that surrounds a body opening or passage. When it contracts, it closes the opening; when it relaxes, it allows material to pass through.

Key Sphincters involved in waste elimination:

1. Internal Anal Sphincter – an involuntary (smooth) muscle that stays contracted by default to prevent accidental passage of feces.

2. External Anal Sphincter – a voluntary (skeletal) muscle that a person can consciously control to delay or allow defecation.

Together, these two sphincters regulate the exit of solid waste (feces) from the body through the anus.

Similarly, the urethral sphincter controls the exit of liquid waste (urine) from the urinary bladder through the urethra.

Why the other options are incorrect:

• Pectoralis – A large chest muscle (pectoralis major/minor) responsible for movements of the shoulder and upper arm. It has no role in waste elimination.

• Rectus – Refers to muscles like Rectus abdominis (the abdominal "core" muscle). While it aids in posture and trunk movement, it is not directly responsible for opening or closing the passage for waste material.

• Gastrocnemius – The large calf muscle at the back of the lower leg, primarily responsible for plantarflexion (pointing the foot downward) and walking. It has no connection to waste elimination.

Summary:
The Sphincter is uniquely designed as a circular gatekeeper muscle. Its controlled contraction and relaxation directly regulate when and how waste material (feces or urine) exits the body, making it the correct answer.