A patient presents with a UTI. The most common causative pathogen is which of the following?

• A. Staphylococcus saprophyticus
• B. Klebsiella pneumoniae
• C. Escherichia coli
• D. Proteus mirabilis

Answer: (C)

The main idea is that urinary tract infections are most commonly caused by a bacterium that is abundant in the gut and has the ability to travel up the urethra to the bladder. Escherichia coli fits this pattern best. It normally resides in the intestinal tract, so it readily colonizes the perineal area and can ascend into the urinary tract, especially in women where the urethra is shorter and closer to the anus. E. coli has specific virulence factors—such as adhesins (type 1 and P fimbriae) that stick to the bladder and ureteral lining, along with other factors that help it resist flushing by urine and invade superficial bladder cells. These traits make it the most frequent cause of both uncomplicated and many complicated UTIs.

Staphylococcus saprophyticus is a recognized cause, particularly in young sexually active women, but it is less common overall than E. coli. Klebsiella pneumoniae and Proteus mirabilis are more often associated with hospital-acquired, catheter-associated, or complicated infections; Proteus also has urease activity that can promote stone formation, which is a feature of specific UTI scenarios rather than the usual community-acquired pattern.