**Core Concept**
The question targets the identification of the most common gram-positive organism causing urinary tract infections (UTIs) in sexually active women. This requires knowledge of the typical pathogens responsible for UTIs in this demographic and their respective characteristics.

**Why the Correct Answer is Right**
Enterococcus faecalis is a gram-positive bacterium commonly found in the gastrointestinal tract and genitourinary tract of humans. It is a frequent cause of UTIs, particularly in sexually active women, due to its ability to adhere to and colonize the uroepithelial cells. Enterococci can also develop resistance to antibiotics, making treatment challenging.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus saprophyticus is a gram-positive bacterium that can cause UTIs, but it is more commonly associated with young, sexually active women. However, it is not the most common gram-positive cause of UTIs in this demographic.
**Option B:** Streptococcus agalactiae (Group B Streptococcus) is a gram-positive bacterium that can cause UTIs, particularly in pregnant women or those with underlying medical conditions. However, it is not the most common gram-positive cause of UTIs in sexually active women.
**Option C:** Lactobacillus is a gram-positive bacterium that is commonly found in the vagina and is typically beneficial. However, some species of Lactobacillus can cause UTIs, but they are not the most common gram-positive causes.

**Clinical Pearl / High-Yield Fact**
Sexually active women are at increased risk of UTIs due to the introduction of bacteria from the genital tract into the urinary tract. The use of spermicides, diaphragms, and other contraceptive methods can also increase the risk of UTIs.

**Correct Answer: C. Lactobacillus is not the most common gram-positive cause of UTI among sexually active women, the correct answer is actually not listed. The correct answer should be D. Enterococcus faecalis.