A 58-year-old man with diabetes mellitus type 2 presents to the emergency department with a 3-day history of fever, dysuria, and flank pain. On examination, he is febrile (38.5°C), tachycardic (110/min), and has costovertebral angle tenderness bilaterally. Laboratory investigations show: serum creatinine 3.2 mg/dL (baseline 1.0 mg/dL), BUN 48 mg/dL, urine output 400 mL/24 hours, urine sodium 8 mEq/L, fractional excretion of sodium (FENa) 0.3%, and urine microscopy shows WBC casts and bacteria. Blood cultures are pending. What is the most likely classification of acute kidney injury in this patient?

Explanation

## Diagnosis: Intrinsic Renal AKI (Acute Tubular Necrosis from Pyelonephritis) ### Clinical Reasoning **Key Point:** This patient has acute pyelonephritis with sepsis causing intrinsic renal injury. The presence of WBC casts and bacteria in urine, along with fever and flank pain, confirms upper urinary tract infection. ### Distinguishing AKI Classification | Feature | Prerenal | Intrinsic Renal | Postrenal | |---------|----------|-----------------|----------| | **FENa** | <1% | >2% | Variable | | **Urine Na** | <20 mEq/L | >40 mEq/L | Variable | | **Urine osmolality** | >500 mOsm/kg | <350 mOsm/kg | Variable | | **Urine casts** | Hyaline | RBC, WBC, granular | Absent | | **Mechanism** | Renal hypoperfusion | Direct tubular/glomerular damage | Obstruction | **High-Yield:** Although FENa is 0.3% (suggesting prerenal), the presence of **WBC casts and bacteriuria** is pathognomonic for intrinsic renal disease (pyelonephritis with acute tubular necrosis). In sepsis and infection, FENa can be low despite intrinsic renal injury due to intense vasoconstriction and tubular dysfunction coexisting. ### Why This Is Intrinsic Renal AKI 1. **WBC casts** = direct evidence of glomerular/tubular inflammation 2. **Bacteriuria + fever + CVA tenderness** = pyelonephritis 3. **Sepsis-induced acute tubular necrosis** = endothelial injury and tubular epithelial damage 4. Low urine output (oliguria) with elevated creatinine = acute renal dysfunction **Clinical Pearl:** In sepsis and infection, FENa may be falsely low because intense renal vasoconstriction and neurohumoral activation occur alongside tubular injury. Do NOT rely on FENa alone when urine casts are present. **Mnemonic: WBC CASTS = Intrinsic Renal Disease** — WBC casts indicate glomerulonephritis, pyelonephritis, or acute interstitial nephritis, all of which are intrinsic renal pathology. ### Management Implications - IV fluid resuscitation (sepsis protocol, not prerenal correction) - Broad-spectrum antibiotics pending culture - Monitor urine output and renal function - Consider ICU admission for sepsis management