Patient presents with oliguria, defined as reduced urine output.  The patient's urine volume is less than 400 ml per 24 hours or less than 0.5 mlkg per hour, indicating potential acute kidney injury AKI or chronic kidney disease CKD.  Assessment includes evaluation of fluid intake, fluid balance, and signs of dehydration such as dry mucous membranes, decreased skin turgor, and hypotension.  Differential diagnosis includes prerenal causes like hypovolemia, heart failure, and renal artery stenosis; renal causes such as acute tubular necrosis ATN, glomerulonephritis, and interstitial nephritis; and postrenal causes including urinary tract obstruction.  Laboratory investigations include serum creatinine, blood urea nitrogen BUN, electrolytes, urinalysis, and potentially renal ultrasound to determine underlying etiology.  Treatment focuses on addressing the underlying cause of oliguria.  Fluid resuscitation with intravenous fluids may be indicated for hypovolemia.  If obstruction is suspected, a Foley catheter may be placed or imaging studies performed.  Monitoring of urine output, fluid balance, and renal function is crucial.  Patient education regarding fluid management and potential complications of oliguria is provided.  ICD-10 code R34.0 will be considered for oliguria, with additional codes for the underlying cause.  CPT codes for evaluation and management, laboratory tests, and procedures will be documented as appropriate.