A 2-year-old patient has been diagnosed with vesicoureteral reflux. The most effective antibiotic for prophylaxis in this age group is:

Rationale: A. Amoxicillin (Amoxil) is sometimes used for prophylaxis in young children, but its efficacy against common urinary pathogens and resistance patterns is lower compared with trimethoprim-sulfamethoxazole. B. Ciprofloxacin (Cipro) is not typically recommended for routine prophylaxis in children due to potential adverse effects on developing cartilage. C. Sulfamethoxazole and trimethoprim (Bactrim) is the preferred first-line antibiotic for prophylaxis in children with vesicoureteral reflux. It is effective against E. coli and other common urinary pathogens, has a favorable safety profile, and is widely used in this age group for long-term low-dose prophylaxis. D. Cephalexin (Keflex) can be used in some cases but is generally considered second-line compared with Bactrim for continuous prophylaxis.