A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 22-year-old sexually active female presents to the emergency department in severe pain. She states that she has significant abdominal pain that seems to worsen whenever she urinates. This seems to have progressed over the past day and is accompanied by increased urge and frequency. The emergency room physician obtains a urinalysis which demonstrates the following: SG: 1.010, Leukocyte esterase: Positive, Protein: Trace, pH: 7.5, RBC: Negative. Nitrite: Negative. A urease test is performed which is positive. What is most likely cause of UTI in this patient?
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A
Klebsiella pneumoniaeIncorrect. Klebsiella pneumoniae produces urease but is a gram-negative rod that reduces nitrate to nitrite (would show nitrite positive on dipstick), inconsistent with this patient's negative nitrite.
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B
Staphylococcus saprophyticusCorrect. Staphylococcus saprophyticus is a urease-producing, nitrite-negative gram-positive coccus and is the second most common cause of uncomplicated UTI in sexually active young women.
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C
Escherichia coliIncorrect. E. coli is the most common UTI pathogen overall but is urease-negative; the positive urease test rules it out here.
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D
Serratia marcescensIncorrect. Serratia marcescens can produce urease but is a nosocomial pathogen typically in catheter-associated UTI, not community-acquired infection in a young woman.
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E
Proteus mirabilisIncorrect. Proteus is urease-positive but typically associated with struvite stones and alkaline urine with nitrite-positive dipstick (gram-negative rod that reduces nitrate); it is uncommon in young sexually active women with uncomplicated UTI.
↑ Tap an answer to reveal the reasoning
Answer: B. A young sexually active woman with dysuria, frequency, urgency, and a urinalysis showing leukocyte esterase positive, nitrite negative, and a positive urease test has uncomplicated cystitis caused by Staphylococcus saprophyticus. S. saprophyticus is the second-most-common cause of community-acquired UTI in sexually active young women (E. coli is first), accounting for 5-15% of cases in this demographic.
Key microbiologic differentiators: S. saprophyticus is gram-positive cocci in clusters, coagulase-negative, novobiocin-resistant (this distinguishes it from S. epidermidis, which is novobiocin-sensitive), and notably produces urease. The urease activity alkalinizes urine (this patient's pH 7.5 fits) and predisposes to struvite stone formation (magnesium ammonium phosphate). However, S. saprophyticus does NOT reduce nitrate to nitrite — only enterobacteriaceae like E. coli, Klebsiella, and Proteus do — which is why the dipstick shows negative nitrite despite a true infection.
Klebsiella pneumoniae is a urease producer but is a gram-negative rod that reduces nitrate (nitrite positive), making it unlikely with a negative nitrite. E. coli is the most common cause overall but is urease-negative. Serratia marcescens is an unusual urease producer in UTI and would be more likely in catheter-associated or nosocomial settings. The combination of urease positivity, negative nitrite, and the young sexually active woman demographic points specifically to S. saprophyticus.
**Why each option:**
**A.** Klebsiella pneumoniae produces urease but is a gram-negative rod that reduces nitrate to nitrite (would show nitrite positive on dipstick), inconsistent with this patient's negative nitrite.
**B.** Correct — Staphylococcus saprophyticus is a urease-producing, nitrite-negative gram-positive coccus and is the second most common cause of uncomplicated UTI in sexually active young women.
**C.** E. coli is the most common UTI pathogen overall but is urease-negative; the positive urease test rules it out here.
**D.** Serratia marcescens can produce urease but is a nosocomial pathogen typically in catheter-associated UTI, not community-acquired infection in a young woman.