Let me discuss your query in some details.
Urine is prepared by kidney and comes down to urinary bladder
for temporary storage, from which it is voided when necessary. Renal pelvis is the collector duct ( a funnel shaped dilatation of upper end of ureter) for receiving the urine from kidney and transfer it to ureter. Most part of the pelvis is inside kidney and called renal pelvis. If some part of the pelvis stays outside the kidney, it is called 'extrarenal pelvis'. [ keep in mind, EXTRARENAL -means outside kidney; don't confuse it as EXTRA RENAL - nothing is extra here].
Extrarenal pelvis is normal for persons with large pelvis. The abnormal variety is due to some obstruction in urinary flow path, that causes stasis of urine inside the pelvis and make it dilated outside kidney.
The pre void and post void volumes are measurement of remaining urine inside urinary bladder before and after micturition respectively. The post void value of 35 ml is higher than normal, that suggest some sort of obstruction. However, the examination should be repeated to make the diagnosis confirmed. As you know, urine is continuously produced in kidney and collected in bladder. If the post void measurement is done more than 5 min after micturition - newly prepared urine may alter the actual result. Sometimes, child may retain some urine voluntarily feeling uneasy in examination environment.
You have not mentioned why you suspected urinary tract
infection(UTI). Was it suggested by your doctor? What were the clinical features (like Fever, Burning sensation during micturition, increased frequency of micturition, pain in lower abdomen etc) ? If UTI is suspected, do urine microscopy, routine tests, culture sensitivity.
Cause of obstruction may be at different levels, as following.
Urethra and bladder outlet: This may be associated with urethral atresia, phimosis, meatal stenosis, anterior and posterior urethral valves, calculus, blood clot, neurogenic bladder meningomyelocele
Ureter: This may be associated with vesicoureteral reflux
( more in female), ureterovesical junction narrowing or obstruction, ureterocele, retrocaval ureter, retroperitoneal tumor
, megaureter-prune belly syndrome
, blood clot or ureteropelvic junction narrowing or obstruction.
Recurrent UTI may occur in such child due to stasis of urine in extrarenal pelvis. There may be scarring and obstruction at pelviureteric junction.
Further investigations like DTPA and DMSA scans may be necessary to exclude such abnormality. If obstruction is not severe (below grade-V), surgical correction is not necessary. You may have to continue antibiotics to combat infections.
However, clinical examination is the first thing to be done before any investigations. Flow of urine, strength of flow, necessity for second voiding -etc are the things to be examined by your doctor. Therefore, I would suggest you to visit your doctor, get your child examined, and take necessary steps as he / she advice.