A 21-year-old sexually active woman with urinary symptoms has a urine culture yielding more than 10^5 colony-forming units per milliliter of a lactose-fermenting gram-negative rod. Which organism is most likely responsible?

• A. Klebsiella pneumoniae
• B. Escherichia coli
• C. Morganella morganii
• D. Proteus mirabilis

Answer: (B)

Uncomplicated urinary tract infections in young sexually active women are most often caused by Escherichia coli. A lactose-fermenting gram-negative rod with significant bacteriuria (≥10^5 CFU/mL) fits the classic pattern of a UTI caused by an Enterobacteriaceae member, and E. coli is by far the most common culprit in this setting. E. coli has virulence factors such as adhesins that help it attach to the bladder and urethra, promoting infection. While Klebsiella pneumoniae also ferments lactose and can cause UTIs, it is less common in healthy young women. Morganella morganii and Proteus mirabilis are non-lactose fermenters, so they don’t fit the lactose-fermenting criterion and are less likely in this scenario. Therefore Escherichia coli is the best choice.