## **Core Concept**
Urinary tract infections (UTIs) can be caused by a variety of bacteria, with Gram-negative organisms being the most common cause. However, in certain demographics and clinical scenarios, Gram-positive organisms can also be responsible. In young, sexually active females, the etiology of UTIs can sometimes involve less common pathogens.

## **Why the Correct Answer is Right**
The correct answer, **Staphylococcus saprophyticus**, is a Gram-positive coccus that is known to cause UTIs, particularly in young, sexually active women. This organism is the second most common cause of UTIs in this demographic, after *Escherichia coli*. *S. saprophyticus* is notable for its association with this specific population and its ability to cause uncomplicated UTIs.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Escherichia coli* is the most common cause of UTIs across many demographics but is typically Gram-negative, not Gram-positive.
- **Option B:** *Klebsiella pneumoniae* is another Gram-negative rod commonly implicated in UTIs, especially in healthcare-associated infections or those with certain risk factors, but it does not match the Gram-positive criterion.
- **Option D:** *Pseudomonas aeruginosa* is a Gram-negative rod that can cause UTIs, particularly in the context of catheter-associated infections or compromised urinary tracts, but it is not Gram-positive.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while *E. coli* is the most common cause of UTIs across many populations, *Staphylococcus saprophyticus* is particularly noteworthy in young, sexually active women as a cause of uncomplicated UTIs. This demographic and clinical context clue can guide the diagnostic consideration towards *S. saprophyticus* as a Gram-positive cause.

## **Correct Answer:** .