3-day IV Antibiotic Treatment Versus 3-day IV Followed by 7-day Oral Antibiotic Treatment for AP in Children

Inclusion Criteria: * Age ≥ 1 month and \< 3 years * For children younger than 3 months, gestational age \> 34 WA * First episode of urinary tract infection * AP defined by temperature ≥ 38°C on day of diagnosis AND positive urinalysis (white cell counts ≥ 10\^4/mL) with a positive urine culture with one Gram- negative bacillus ≥ 104 UFC/mL. The child temperature will have to be measured with a thermometer according to the French national recommendations \[Health Insurance website (AMELI ;see: - https://www.ameli. fr/assure/sante/bons-gestes/soins/prendre-temperature); HAS (see: https://www.has-sante. fr/jcms/c\_2674284/fr/prise-en-charge-de-la-fievre-chez-l-enfant)\]. * Initial treatment by either ceftriaxone AND/OR amikacin * Outpatient or hospitalised Non-inclusion Criteria: * Urine collected by bag * Urine culture growing more than one dominant bacterium (cf section 6.2 of the protocol) * Catheter-associated acute pyelonephritis * Known congenital anomalies of the kidney and genitourinary tract (other than vesicoureteral reflux and pyelocaliceal dilatation \< 10 mm) * Previous surgery of the genitourinary tract (except circumcision in male children) * Abnormal renal function for age and weight (defined by a serum creatinine \>40µmol/L before 1 year and \>75µmol between 1 year et 3 years) * Known immunocompromising condition (e.g., HIV, primary immunodeficiency, sickle cell disease, use of chronic corticosteroids or other immunosuppressive agents) * Antibiotic prophylaxis for any reason OR antibiotic treatment in the last 7 days (except treatment administered for the AP) * Known hypersensitivity to at least one of the active substances /excipients: ceftriaxone (including other cephalosporins and other beta-lactams) and amikacin (including other aminoglycosids). * Known hypersensitivity to at least one of the active substances /excipients: cotrimoxazole (=sulfamethoxazole/trimethoprim) (including other drugs containing sulfonamides) and cefixime (including other cephalosporins) * Known blood dyscrasias (megaloblastic haematopoiesis) * Known severe hepatic insufficiency * Known G6PD deficiency * No written consent from holders of parental authority * Non-affiliation to a social security system (as beneficiary or entitled person) * Children whose follow-up is not carried out in the centre * Participation in another interventional or minimal risk trial Randomization criteria : * Three days of taking antibiotics (IV or IM) (no interruption or discontinuation) * Positive urine culture with Gram negative bacillus ≥ 10\^4 UFC/mL * Favorable clinical outcome at day of randomization (D2 or D3) defined by temperature \< 38°C at day of randomization and absence of fever measured \> 38°C for at least 12 hours AND no abdominal pain AND no feeding problem AND investigator agreement * No renal abscess AND congenital anomalies of the kidney and genitourinary tract (other than vesicoureteral reflux and pyelocaliceal dilatation \< 10 mm) on the renal ultrasound performed between D0 and day of randomization * No more than 1 type of dominant bacteria on the urine culture * Sensitivity to the initial antibiotic treatment * Sensitivity to cefixime OR cotrimoxazole