Inclusion Criteria:  3 months (corrected age) to 18 years  3 or more of the following present: Fever, Vomiting, Rigors, History of recurrent UTI, Urological abnormalities, Tachycardia Abnormal urinary dipstick leucocyte esterase >1+ or nitrite positive OR ≥5 White Blood Cells (WBCs) per high-power field in centrifuged urine OR≥ 10 White Blood Cells (WBCs) per mm3 in uncentrifuged urine and bacteriuria with any bacteria per high-power field ED clinician determines the child requires treatment with IV antibiotics  Exclusion Criteria:  Sepsis (requiring inotropic support or more than 20mls/kg of fluid bolus) Known allergy to the once daily study drug options (gentamicin or ceftriaxone or amikacin) If the patient has another co-existing condition which requires (based on established evidence-based guidelines) more than 1 dose of IV antibiotics eg meningitis Impaired renal function (abnormal creatinine at ED presentation or known renal impairment or renal transplant patients) Unrepaired posterior urethral valves Indwelling stent and fever Patients with known pelvic ureteric junction obstruction (PUJ obstruction) where pyonephrosis (pus in the renal pelvis) is a potential diagnosis and cannot be ruled out (ultrasound ruling out pyonephrosis prior to randomisation) Patients with known vesico ureteric junction obstruction (VUJ obstruction) where pyonephrosis (pus in the renal pelvis) is a potential diagnosis and cannot be ruled out (require ultrasound ruling out pyonephrosis prior to randomisation)