**Core Concept**
The most common cause of Urinary Tract Infections (UTIs) in sexually active young women is often related to the introduction of bacteria into the urinary system, typically through sexual activity. **Escherichia coli (E. coli)** is a common culprit due to its prevalence in the gastrointestinal tract and its ability to colonize the genital area.

**Why the Correct Answer is Right**
**E. coli** is responsible for the majority of UTIs in this demographic due to its virulence factors, such as adhesins that allow it to adhere to the uroepithelial cells, and its ability to outcompete other flora. Sexual activity can facilitate the introduction of **E. coli** from the rectal area to the urethra, leading to infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while other bacteria can cause UTIs, they are not as commonly associated with sexually active young women as **E. coli**.
**Option B:** Incorrect as it is not specified, but typically, other bacteria like **Klebsiella** or **Proteus** are less common causes.
**Option C:** Incorrect because, although **Staphylococcus saprophyticus** can cause UTIs in young women, it is less common than **E. coli**.
**Option D:** Incorrect as it is not the most common cause in this demographic.

**Clinical Pearl / High-Yield Fact**
Remember, **E. coli** is the most common cause of UTIs in otherwise healthy individuals, and its identification is crucial for targeted antibiotic therapy.

**Correct Answer:** D. Escherichia coli