**Core Concept**
The most common cause of urinary incontinence in postmenopausal women is related to the weakening of the pelvic floor muscles and the urethral sphincter, leading to a decrease in urethral closure pressure. This condition is often attributed to the loss of estrogenic effects on the genitourinary system.

**Why the Correct Answer is Right**
The correct answer is related to the loss of estrogenic effects on the genitourinary system. Estrogen plays a crucial role in maintaining the health of the urethral mucosa and the smooth muscle of the urethral sphincter. Decreased estrogen levels post-menopause lead to thinning of the urethral mucosa, which compromises the urethral closure pressure. This results in stress urinary incontinence, which is the most common type of urinary incontinence in postmenopausal women.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because stress urinary incontinence is more common than urge incontinence in postmenopausal women. While urge incontinence can occur at any age, it is more commonly associated with neurological conditions or bladder outlet obstruction.
* **Option B:** This option is incorrect because detrusor overactivity is more commonly associated with urge incontinence, which is not the most common cause of urinary incontinence in postmenopausal women.
* **Option C:** This option is incorrect because urethral prolapse is a rare condition that can cause urinary incontinence, but it is not the most common cause in postmenopausal women.

**Clinical Pearl / High-Yield Fact**
The loss of estrogenic effects on the genitourinary system post-menopause can lead to a range of symptoms, including vaginal dryness, decreased libido, and urinary incontinence. Estrogen replacement therapy can be effective in managing these symptoms.

**Correct Answer:** C. Urethral sphincter weakness due to loss of estrogenic effects.