A 35-year-old woman from Mumbai presents with a 3-week history of fever, malaise, and loose stools. She was treated empirically with ciprofloxacin 500 mg twice daily for 5 days by a local practitioner 2 weeks ago, but fever persisted. On examination, she is febrile (38.8°C), has mild hepatomegaly, and a faint maculopapular rash on the trunk. Blood culture obtained on day 21 of illness grows *Salmonella typhi* susceptible to ceftriaxone but resistant to fluoroquinolone and chloramphenicol. What is the most likely reason for treatment failure with ciprofloxacin?