Right loin pain, similar to previous renal colic but worse
At the lower pole of the right kidney there is a perinephric fluid collection. This has decompressed into the peritoneal space and a fluid tract passing through the retroperitoneum and the breach of the posterior peritoneum is evident.
The likely mechanism is a breach of the collecting system due to increased pressure caused by ureteric obstruction. This has allowed the hydronephrosis to resolve but has resulted in this perinephric collection and breach into the peritoneal space - an auto-nephrostomy!