**Core Concept**
The internal anal sphincter (IAS) is a part of the anal sphincter complex, which is essential for maintaining fecal continence. The IAS is a smooth muscle layer that is innervated by the sympathetic and parasympathetic nervous systems.

**Why the Correct Answer is Right**
The correct answer is related to the embryological development of the anal sphincter complex. The IAS is derived from the mesodermal cells of the cloaca, which eventually give rise to the smooth muscle layer of the IAS. The IAS is a continuation of the smooth muscle layer of the rectum and is located below the pectinate line. It is innervated by the sympathetic nerves from the inferior hypogastric plexus and the parasympathetic nerves from the pelvic splanchnic nerves.

**Why Each Wrong Option is Incorrect**
**Option A:** The external anal sphincter (EAS) is a skeletal muscle layer that is innervated by the somatic nerves, whereas the IAS is a smooth muscle layer.
**Option B:** The anal mucosa is a thin layer of epithelium that lines the anal canal, but it is not a part of the anal sphincter complex.
**Option C:** The levator ani muscle is a part of the pelvic floor muscles, but it is not a part of the anal sphincter complex.

**Clinical Pearl / High-Yield Fact**
The IAS is responsible for maintaining fecal continence by contracting and relaxing in response to changes in rectal pressure. Damage to the IAS can lead to fecal incontinence, which is a common complication of anal surgery.

**Correct Answer:** D. The anal sphincter complex.