Final answer:
The muscle that separates the internal from the external sphincter is made of skeletal muscle fibers, which are voluntary, whereas the internal sphincter consists of involuntary smooth muscle fibers (option D).
Step-by-step explanation:
The muscle that separates the internal sphincter from the external sphincter is made up of skeletal muscle fibers. These fibers allow the outer rectal muscle to be under voluntary control, meaning we can consciously control the contraction and relaxation of this sphincter.
In contrast, the internal rectal sphincter consists of smooth muscle fibers, which are under involuntary control and are responsible for maintaining constriction of the sphincter except during defecation.
It is also important to note that smooth muscle fibers differ from skeletal and cardiac muscle fibers in their structure. Smooth muscle fibers are spindle-shaped, have a single nucleus, lack striations, and contain actin and myosin contractile proteins.
Unlike skeletal and cardiac muscles, which are striated due to their regular arrangement of sarcomeres, smooth muscles sustain smooth, involuntary, and often near-permanent contractions without these striations.
Hence, the answer is option D.