A urine culture shows greater than 100,000 CFU/mL of Escherichia coli. This finding most strongly supports which diagnosis?</n>

• A. Urinary tract infection
• B. Contamination of the specimen
• C. A noninfectious bladder disorder
• D. A kidney stone

Answer: (A)

A high level of bacteria in the urine coming from a clean specimen, especially when a single organism like Escherichia coli is predominant, strongly points to an infection of the urinary tract. The threshold of 10^5 colony-forming units per milliliter is a conventional sign of significant bacteriuria, meaning the bacteria are actively growing in the urinary tract rather than merely being introduced by the collection process. Escherichia coli is the most common culprit in UTIs, so its predominance in the culture reinforces this interpretation. A noninfectious bladder disorder would not produce such a significant bacteriuria. While a kidney stone can be associated with UTIs, the culture result alone does not indicate a stone. Thus, the finding most strongly supports a urinary tract infection.