A 28-year-old woman from Delhi presents with a 10-day history of sustained fever, headache, and abdominal pain. Blood culture grows Salmonella typhi sensitive to fluoroquinolones, third-generation cephalosporins, and azithromycin, but resistant to chloramphenicol and trimethoprim-sulfamethoxazole. What is the drug of choice for treatment of this enteric fever?