Time to play!  : the answer
Great participation to this interesting case of pyonephrosis in a cat!
The presence of echogenic material in the renal pelvis and possibly ureters is a key finding in suspecting pus.
If the animal is not responding to medical management, a pyelocentesis might be the recommended next step.
For more examples of this disease, please consult this link on the Small Animal Ultrasonography website: https://smallanimalultrasonography.com/pyonephrosis-in-cats/