Urinary tract infections – second most common cause of cystitis in healthy sexually active women (after Escherichia coli) Which pathogen most commonly causes cystitis after Escherichia coli in otherwise healthy, sexually active young women?

Introduction / Context:
Acute uncomplicated cystitis is most frequently caused by uropathogenic E. coli, but epidemiology also emphasizes a consistent second-place culprit in sexually active young women: Staphylococcus saprophyticus.

Given Data / Assumptions:

• Population: healthy, sexually active females.
• Clinical syndrome: acute cystitis (dysuria, frequency, urgency).
• Common uropathogens vary by host factors and setting.

Concept / Approach:
S. saprophyticus is a coagulase-negative, novobiocin-resistant staphylococcus with tropism for the urinary tract. It adheres to uroepithelium and is prevalent in community-acquired cystitis in young women. Other organisms (Proteus, Klebsiella, Enterococcus) are important but less common in this demographic absent complicating factors.

Step-by-Step Solution:
Identify the demographic and syndrome.Recall hallmark association: S. saprophyticus second to E. coli.Select “Staphylococcus saprophyticus.”

Verification / Alternative check:
Outpatient UTI studies consistently list S. saprophyticus after E. coli among sexually active young women.

Why Other Options Are Wrong:

• Proteus mirabilis: common in complicated UTIs and catheterized patients; urease-positive stones.
• Pseudomonas aeruginosa: nosocomial, associated with devices/antibiotics.
• Klebsiella pneumoniae: notable cause but less frequent than S. saprophyticus in this group.
• Enterococcus faecalis: seen in certain settings but not the typical second most common in healthy young women.

Common Pitfalls:
Assuming hospital pathogen rankings apply to community-acquired UTIs in otherwise healthy hosts.

Final Answer:
Staphylococcus saprophyticus.