**Core Concept:** The female genital system consists of a complex network of muscles, ligaments, and structures that work together to maintain urinary and fecal continence, as well as providing support to the pelvic organs. The vaginal sphincter is a crucial component of this system, and understanding its composition is essential for proper assessment and management of pelvic floor dysfunction.

**Why the Correct Answer is Right:** The vaginal sphincter is primarily composed of smooth muscle fibers, specifically the external anal sphincter and the internal urethral sphincter. These structures are primarily responsible for controlling the passage of urine and feces through the anus and urethra.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Smooth Muscle)**: Smooth muscle fibers are part of the vaginal sphincter, not an exclusion.
B. **Option B (Erector Spinae)**: Erector spinae is a group of spinal muscles involved in posture and movement, not part of the vaginal sphincter.
C. **Option C (Rectus Abdominis)**: Rectus abdominis is a muscle of the abdominal wall, unrelated to the vaginal sphincter.
D. **Option D (Bulbospongiosus)**: Bulbospongiosus is a muscle of the perineum, not a component of the vaginal sphincter.

**Clinical Pearl:** Proper identification of the vaginal sphincter components is essential in the diagnosis and management of pelvic floor disorders. A thorough physical examination, including assessment of the anus, urethra, and vagina, is crucial for accurate diagnosis and treatment planning.