Which structure is involved in the innervation of the internal anal sphincter?

The correct involvement in the innervation of the internal anal sphincter is through sympathetic fibers, which originate primarily from the lumbar region of the spinal cord. The internal anal sphincter is a smooth muscle that functions involuntarily, and its contraction and relaxation are under autonomic control.

Sympathetic innervation plays a significant role in maintaining the tone of the internal anal sphincter, allowing for continence. These fibers travel to the sphincter via the hypogastric nerve, facilitating the contraction of the sphincter when necessary.

The other structures listed do not have a primary role in the innervation of the internal anal sphincter. The pudendal nerve is primarily responsible for the somatic control of the external anal sphincter and sensory innervation to the perineum. The vagus nerve is associated with parasympathetic innervation elsewhere in the body, primarily focused on the thoracic and abdominal viscera, and does not directly control the anal sphincters. The phrenic nerve innervates the diaphragm, playing no role in anal sphincter function. Thus, sympathetic fibers are indeed the key players in regulating the internal anal sphincter's activity.