Which organism is novobiocin-resistant and a common cause of urinary tract infections in sexually active young women?

• A. Staphylococcus aureus
• B. Staphylococcus epidermidis
• C. Staphylococcus saprophyticus
• D. Streptococcus pyogenes

Answer: (C)

The key idea is using antibiotic susceptibility to differentiate common staphylococcal species and connect that pattern to a typical clinical scenario. Staphylococcus saprophyticus is notable for being resistant to novobiocin, whereas most other Staphylococcus species are inhibited by it. This resistance pattern, combined with its prominence as a cause of uncomplicated urinary tract infections in young, sexually active women, makes it the most likely culprit in this context.

So, organisms like Staphylococcus aureus and Staphylococcus epidermidis are generally susceptible to novobiocin, and Streptococcus pyogenes is not a common cause of UTIs in this demographic and does not fit the novobiocin-resistance clue. The combination of novobiocin resistance and the patient population points to Staphylococcus saprophyticus as the best answer.