**Core Concept**
The internal anal sphincter is a component of the anal sphincter complex, playing a crucial role in maintaining **continence** and regulating the passage of stool. It is formed by a thickening of the **internal circular smooth muscle layer** of the **anal canal**. This structure is essential for the fine-tuned control of defecation.

**Why the Correct Answer is Right**
The correct answer is related to the composition of the internal anal sphincter, which is made up of smooth muscle. This smooth muscle is **involuntary**, meaning its contraction and relaxation are not under conscious control, and it is responsible for the **basal tone** of the anal canal, preventing involuntary loss of stool.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not accurately describe the composition of the internal anal sphincter.
**Option B:** Incorrect as it refers to a different component of the anal sphincter complex.
**Option C:** Incorrect because it is not related to the structure of the internal anal sphincter.

**Clinical Pearl / High-Yield Fact**
The internal anal sphincter's function can be compromised in conditions such as **anal fissures** or **fecal incontinence**, leading to significant quality of life issues. Understanding its structure and function is crucial for managing these conditions.

**Correct Answer:** D. Smooth muscle.