A 21-year-old sexually active woman with urinary symptoms and a lactose-fermenting gram-negative rod in culture most likely etiologic agent is

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

Answer: (A)

The key idea is that urinary tract infections in young sexually active women are most commonly caused by Escherichia coli. It’s a lactose-fermenting Gram-negative rod within the Enterobacteriaceae family, which fits the culture finding described. E. coli originates from the gut and typically reaches the urinary tract by ascending from the periurethral area, an infection pathway that sexual activity can promote. Its virulence factors, such as type 1 fimbriae, help it adhere to bladder mucosa and establish infection, making it the frequent culprit in uncomplicated cystitis. While other lactose-fermenting organisms like Klebsiella can cause UTIs, they are less common in this clinical scenario. Non-lactose-fermenters like Proteus mirabilis or Morganella morganii are associated with different patterns (Proteus with stones, Morganella less common). So the lactose-fermenting pattern in this patient most strongly points to Escherichia coli.