Case Report: Looks can deceive: acute postoperative progressive renal dysfunction and hyperkalemia without significant hydronephrosis.

Introduction: Hydronephrosis is typically considered a hallmark radiographic finding of urinary tract obstruction, expected to develop within 7 days of onset. However, this timeline may not apply universally. We report a rare case of delayed-onset hydronephrosis in the context of borderline end-stage renal dysfunction following radical nephrectomy in a patient with metastatic renal cell carcinoma (RCC), emphasizing the limitations of imaging in the early diagnosis of obstruction. Case presentation: A 59-year-old male patient with metastatic renal cell carcinoma underwent right cytoreductive nephrectomy for an 8.5-cm renal mass with significant retroperitoneal lymphadenopathy and pulmonary metastases. Preoperative imaging showed mild left-sided hydronephrosis, and baseline renal function was preserved [estimated glomerular filtration rate (eGFR) 68 mL/min/1.73 m Discussion: This case illustrates the potential for clinically significant obstruction to occur in the absence of early hydronephrotic changes on imaging. We hypothesize that hyperacute progressive obstruction may lead to rapid intrarenal pressure stabilization, limiting capsular stretch and delaying radiographic findings. The absence of pain and significant radiographic hydronephrosis contributed to diagnostic delay. Conclusions: The absence of hydronephrosis on imaging cannot exclude obstruction, particularly in patients with a solitary functional kidney and high-risk features such as retroperitoneal malignancy. Early clinical deterioration should prompt a high index of suspicion and further diagnostic evaluation to prevent irreversible renal injury.