What is the most common organism causing urinary tract infections in adults?</n>

• A. Escherichia coli
• B. Staphylococcus aureus
• C. Pseudomonas aeruginosa
• D. Klebsiella pneumoniae

Answer: (A)

Escherichia coli is the most common cause of urinary tract infections in adults because it normally lives in the gut and can readily reach the urinary tract. It’s equipped with factors that help it cling to and invade the bladder lining, making ascent from the urethra into the bladder easy. This combination of being plentiful in the gut and having tools to adhere to urinary tract tissues means E. coli accounts for the large majority of UTIs in adults, including many in older adults.

Staphylococcus aureus tends to cause infections stemming from skin or bloodstream sources and is less typically a primary bladder pathogen. Pseudomonas aeruginosa is more associated with hospital settings and complicated cases, such as those with indwelling devices or long-term catheterization, where it can appear but is not as common overall. Klebsiella pneumoniae does cause UTIs, but it occurs less frequently than E. coli.

So, the best answer is Escherichia coli because it is by far the most frequent organism causing UTIs in adults.