A 24-year-old female presents with 2 days of dysuria, urinary frequency, and urgency. She is afebrile and has no flank pain. Urinalysis shows: Positive leukocyte esterase Negative nitrites 15–20 WBCs/high-power field Which of the following is the most likely causative organism? A. Escherichia coli B. Proteus mirabilis C. Klebsiella pneumoniae D. Staphylococcus saprophyticus E. Pseudomonas aeruginosa
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
The answer is D. Staphylococcus saprophyticus, and the textbook sources confirm this clearly. Here is a full breakdown:
Answer: D. Staphylococcus saprophyticus
Why this patient points to S. saprophyticus
| Clue | Significance |
|---|---|
| 24-year-old female | Classic demographic - young, sexually active women aged 16-35 |
| Dysuria, frequency, urgency | Lower UTI (cystitis), not upper tract |
| Afebrile, no flank pain | Uncomplicated UTI - rules out pyelonephritis |
| Positive leukocyte esterase | Confirms pyuria (infection is real) |
| Negative nitrites | Key discriminator - gram-negative rods (E. coli, Klebsiella, Proteus) reduce nitrates to nitrites; S. saprophyticus does NOT |
| 15-20 WBCs/hpf | Significant pyuria, consistent with bacterial cystitis |
The negative nitrite with positive leukocyte esterase in a young woman is the classic exam signal for S. saprophyticus.
Why the other organisms are wrong
- A. E. coli - Most common overall cause of UTI (~75%), but it IS a gram-negative rod that DOES produce nitrites. The nitrite would be positive.
- B. Proteus mirabilis - Also gram-negative, produces nitrites, and is more associated with urease activity, struvite stones, and complicated/recurrent UTIs.
- C. Klebsiella pneumoniae - Gram-negative, produces nitrites, more common in complicated or hospital-acquired UTIs.
- E. Pseudomonas aeruginosa - Gram-negative, associated with complicated UTIs, catheter-associated infections, and immunocompromised/hospitalized patients. Not a community pathogen in healthy young women.
Textbook confirmation
Per Tintinalli's Emergency Medicine (Table 91-2), the etiologic agents in uncomplicated UTI are:
- E. coli (75%)
- Staphylococcus saprophyticus (second most common)
- Klebsiella pneumoniae
- Enterococcus faecalis
Per Jawetz, Melnick & Adelberg's Medical Microbiology: "S. saprophyticus is a relatively common cause of urinary tract infections in young women, although it rarely causes infections in hospitalized patients."
Per Red Book 2021: "Some species of CoNS are associated with urinary tract infection, including Staphylococcus saprophyticus in adolescent females and young adult women, often after sexual intercourse."
Key Facts to Remember for Boards
- S. saprophyticus is coagulase-negative, novobiocin-resistant (distinguishes it from S. epidermidis, which is novobiocin-sensitive)
- It is a gram-positive coccus, so it does NOT convert nitrate to nitrite
- It has a seasonal peak in late summer/fall
- Treatment: trimethoprim-sulfamethoxazole or nitrofurantoin (same as uncomplicated cystitis)
This is a shared conversation. Sign in to Orris to start your own chat.