pyelovenous backflow

noun

py·​elo·​ve·​nous back·​flow -ˈvē-nəs-ˈbak-ˌflō 

: drainage of fluid from the renal pelvis of a kidney into the renal venous system under certain conditions (as in retrograde pyelography) in which abnormal amounts of pressure occur in a direction opposite to normal

called also pyelovenous reflux