A 20-year-old pregnant woman (gravida II, para I) complains of lower pelvic discomfort, fever, and pain during urination for the past 2 days. She also reports seeing blood in her urine. In the patient, which of the following would be the most likely etiologic agent?

Gram-negative bacteria from the feces, most commonly E. coli, cause 80% of upper and lower urinary tract infections in females. Asymptomatic bacteriuria occurs in 10% of pregnant women, one fourth of whom go on to develop acute pyelonephritis. Less common causes of acute cystitis include Proteus vulgaris (choice C), Pseudomonas aeruginosa (choice D), and Enterobacter sp. (choice A). Most cases of cystitis respond well to treatment with antimicrobial agents. Stromal edema, hemorrhage, and a neutrophilic infiltrate of variable intensity are typical of acute cystitis.Diagnosis: Cystitis, acute