A 27-year-old woman presents with a three-day history of painful urination, increased urinary frequency, and suprapubic discomfort. She denies fever, flank pain, or vaginal discharge. Her past medical history is unremarkable, and she takes no medications. She is sexually active with one partner and uses oral contraceptive pills. She does not smoke or drink alcohol. Her temperature is 98.7°F (37.0°C), heart rate is 78 beats/min, and blood pressure is 122/78 mmHg. On physical examination, there is mild suprapubic tenderness but no costovertebral angle tenderness. A urinalysis reveals pyuria and positive nitrites.
Which of the following is the most likely causative organism?
- A) Enterococcus faecalis
- B) Escherichia coli
- C) Klebsiella pneumoniae
- D) Proteus mirabilis
- E) Staphylococcus saprophyticus
B) Escherichia coli
Escherichia coli is the most common cause of uncomplicated urinary tract infections (UTIs), particularly in young women. Positive nitrites on urinalysis suggest infection with a gram-negative enteric organism such as E. coli, though gram-positive bacteria also can produce nitrites.
Answer choice A: Enterococcus faecalis, is incorrect. Enterococcus can cause UTIs but is less common in young women with uncomplicated infections.
Answer choice C: Klebsiella pneumoniae, is incorrect. Klebsiella is a less frequent cause of UTIs but can occur in hospitalized patients or those with catheters.
Answer choice D: Proteus mirabilis, is incorrect. Proteus is associated with struvite stone formation and alkaline urine due to urease production. It is a less common cause of uncomplicated UTIs.
Answer choice E: Staphylococcus saprophyticus, is incorrect. Staphylococcus saprophyticus can cause UTIs in sexually active young women, but E. coli remains the most common cause.
Key Learning Point
Escherichia coli is the most common cause of uncomplicated urinary tract infections and is associated with positive nitrites on urinalysis.
