**Core Concept**
The pelvic floor muscles play a crucial role in maintaining urinary continence by providing support to the urethra and bladder neck. The most significant muscle in this context is the one that surrounds the urethra and helps to prevent involuntary contraction of the urethral sphincter.

**Why the Correct Answer is Right**
The correct answer is the external urethral sphincter, which is a skeletal muscle that surrounds the urethra and helps to maintain urinary continence. The external urethral sphincter is innervated by the pudendal nerve and is responsible for voluntary control of urination. When the external urethral sphincter contracts, it helps to prevent the involuntary contraction of the urethra and maintains continence. A significant tear in this muscle, as revealed by the MRI examination, would likely result in urinary incontinence.

**Why Each Wrong Option is Incorrect**
* **Option A:** The levator ani muscle is a major muscle of the pelvic floor, but it is not primarily responsible for maintaining urinary continence. It does provide support to the urethra and bladder neck, but its primary function is to support the pelvic organs.
* **Option B:** The obturator internus muscle is not directly involved in maintaining urinary continence. It is a muscle of the pelvic wall that helps to rotate the femur and stabilize the hip joint.
* **Option C:** The anal sphincter is a muscle that surrounds the anus and helps to maintain fecal continence, but it is not primarily responsible for maintaining urinary continence.

**Clinical Pearl / High-Yield Fact**
A significant tear in the external urethral sphincter, as in the case of a pelvic floor injury, can result in urinary incontinence. This is because the external urethral sphincter provides voluntary control of urination and helps to prevent involuntary contraction of the urethral sphincter.

**Correct Answer:** C.