**Core Concept**
The vaginal sphincter is a complex structure composed of muscles and fascial layers that provide support and maintain continence in females. Understanding the anatomy and components of the vaginal sphincter is crucial in diagnosing and managing pelvic floor disorders.

**Why the Correct Answer is Right**
The correct answer is the component that does not belong to the vaginal sphincter. The vaginal sphincter primarily consists of the external urethral sphincter, the external anal sphincter, and the pubococcygeus muscle (also known as the puborectalis muscle). These muscles work together to maintain continence and support the pelvic organs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the external urethral sphincter is indeed a component of the vaginal sphincter.
**Option B:** This option is incorrect because the external anal sphincter is also a part of the vaginal sphincter complex.
**Option C:** This option is incorrect because the pubococcygeus muscle (or puborectalis muscle) is a key component of the vaginal sphincter.

**Clinical Pearl / High-Yield Fact**
The vaginal sphincter is often assessed during pelvic floor examinations, and its integrity is crucial in diagnosing and managing conditions such as pelvic organ prolapse and fecal incontinence. The presence of vaginal sphincter defects can significantly impact a patient's quality of life.

**Correct Answer: D.**