A 28-year-old woman from Delhi presents to the emergency department with acute onset dysuria, frequency, and suprapubic pain for 2 days. She denies fever, flank pain, or vaginal discharge. Urinalysis shows pyuria (>10 WBC/hpf), bacteriuria, and nitrites. Urine culture is pending. Blood pressure is 120/80 mmHg, temperature 37.2°C. On direct Gram stain of uncentrifuged urine, Gram-negative rods are seen. Which of the following is the most likely organism and the most appropriate empirical antibiotic for uncomplicated acute cystitis in this patient?