Question

Smooth muscles are found in;
A. External Anal sphincter.
B. Internal anal Sphincter.
C. Internal Urethral sphincter.
D. Both b and c.

Answer 1

Smooth muscle is a form of muscle tissue that is used to apply pressure to vessels and organs through different systems. They are not striated, branched, or singly nucleated, and they are involuntarily regulated. These cells have actin and myosin fibres that pass through the cell and are protected by a protein matrix. They're embedded in the walls of hollow organs like the stomach, intestines, urinary bladder, and uterus, as well as passageways like the circulatory system's arteries and veins, and the respiratory, urinary, and reproductive systems' tracts.

Complete explanation:
Option A: The external anal sphincter is a skeletal muscle tube that surrounds the anal canal's inferior part. It is one of three structures that regulate defecation in the anal sphincter complex.
Option A is incorrect:
Option B: Smooth muscle makes up the internal anal sphincter, which provides $80-85\%$ of resting pressure. The external anal sphincter (EAS) is a voluntary muscle that contracts in response to rectal distension, allowing the individual to postpone defecation until a more convenient time.
Option B alone is incorrect.
Option C: At the junction of the urethra and the urinary bladder, the internal sphincter muscle of the urethra is situated at the bladder's inferior end and the urethra's proximal end. Since the internal sphincter is made of smooth muscle and is a continuation of the detrusor muscle, it is under involuntary or autonomic regulation.
Option C alone is incorrect.
Option D: The internal anal sphincter is made up of smooth muscle and provides $80-85\%$ of resting strain. The external anal sphincter (EAS) is a voluntary muscle that contracts in response to rectal distension, allowing a person to delay defecation until a later time. The internal sphincter muscle of the urethra is located at the junction of the urethra and the urinary bladder, at the bladder's inferior end and the urethra's proximal end. The internal sphincter is governed by involuntary or autonomic control since it is made of smooth muscle and is a continuation of the detrusor muscle.
Option D is correct.

Hence, Option D is the correct answer.

Note:
An anal sphincter is a group of muscles at the end of the rectum that covers the anus and regulates stool release, allowing continence to be maintained. There are two types of sphincter muscles: internal and external. The internal anal sphincter is a smooth muscle ring that covers the anal canal for around $2.5-4.0cm$. Its inferior boundary is in contact with, but distinct from, the external anal sphincter. The external anal sphincter is an elliptical flat plane of skeletal muscle fibres that adheres to the skin around the anus's edge.