## **Core Concept**
The **sphincter urethrae**, also known as the urethral sphincter, is a muscle that plays a crucial role in controlling the flow of urine out of the body by encircling the urethra. It is essential for maintaining urinary continence. The urethral sphincter is composed of both smooth and skeletal muscle components, with the external urethral sphincter being primarily skeletal muscle.

## **Why the Correct Answer is Right**
The correct answer is related to the composition and function of the sphincter urethrae. Typically, the **external sphincter urethrae** is composed of **skeletal muscle** (striated muscle), which is under voluntary control, allowing for the conscious control of urination. This muscle is crucial for the prevention of involuntary loss of urine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If option A states that the sphincter urethrae is composed of skeletal muscle, this is a true statement regarding the external urethral sphincter, which is indeed made of skeletal muscle.
- **Option B:** If option B mentions that the sphincter urethrae is involved in maintaining urinary continence, this is also true as its primary function is to prevent the involuntary flow of urine.
- **Option C:** Without specific details on option C, we can infer based on common knowledge that if it accurately describes a characteristic of the sphincter urethrae, it would be a true statement.
- **Option D:** If option D incorrectly states that the sphincter urethrae is primarily made of smooth muscle (when referring to the external sphincter), this would be incorrect because the external sphincter urethrae is primarily skeletal muscle.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **external urethral sphincter** is a skeletal muscle and is under voluntary control. Damage to this muscle or its innervation can lead to urinary incontinence.

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