Community-acquired urinary tract infection — The most common etiological agent in otherwise healthy outpatients is:

Introduction / Context:
Urinary tract infections (UTIs) are among the most frequent bacterial infections in the community, especially in women. Accurate empirical therapy depends on knowing the most likely pathogen before culture results return.

Given Data / Assumptions:

• Community-onset UTI in otherwise healthy adults.
• No complicating factors such as catheters or recent hospitalization (which shift etiology).
• Standard epidemiology of uropathogens applies.

Concept / Approach:
Escherichia coli causes the majority of uncomplicated community-acquired UTIs due to virulence factors like P fimbriae, adhesins, and the ability to colonize the periurethral area from gut reservoirs. While Klebsiella, Proteus, and Citrobacter can cause UTIs, their prevalence is significantly lower in community settings compared with E. coli.

Step-by-Step Solution:

Verification / Alternative check:
Large surveillance studies consistently report E. coli as the leading cause of outpatient UTIs, supporting this choice.

Why Other Options Are Wrong:

• Klebsiella/Proteus/Citrobacter: occasional causes, more frequent in complicated or healthcare-associated infections.

Common Pitfalls:
Assuming the same distribution applies to catheter-associated or nosocomial UTIs; those settings have broader etiologies and resistance patterns.

Final Answer:
Escherichia coli