A 58-year-old man with diabetes mellitus type 2 presents to the emergency department with a 3-day history of fever, cough, and dyspnea. On examination, he is hypotensive (BP 88/54 mmHg), tachycardic (HR 118/min), and has bilateral crackles on auscultation. Laboratory investigations reveal: serum creatinine 3.2 mg/dL (baseline 1.0 mg/dL), BUN 68 mg/dL, urine output 0.3 mL/kg/hr, urine sodium 8 mEq/L, urine osmolality 580 mOsm/kg. Chest X-ray shows bilateral infiltrates. What is the most likely type of acute kidney injury in this patient?

Explanation

## Clinical Diagnosis: Prerenal AKI (Volume Depletion / Hypoperfusion) ### Key Findings Supporting Prerenal AKI **High-Yield:** This patient has sepsis-induced hypoperfusion causing prerenal azotemia. The urinary indices are the gold standard for classifying the type of AKI: | Feature | Finding | Significance | |---------|---------|---------------| | **Urine sodium** | 8 mEq/L (< 20 mEq/L) | Tubules intact → avid sodium reabsorption | | **Urine osmolality** | 580 mOsm/kg (> 500 mOsm/kg) | Intact concentrating ability → ADH response preserved | | **FENa** | < 1% (estimated) | Classic prerenal pattern | | **BUN:Cr ratio** | 68:3.2 ≈ 21:1 | > 20 supports prerenal physiology | | **Clinical context** | Sepsis with hypotension | Effective circulating volume depletion | | **Urine output** | 0.3 mL/kg/hr (oliguria) | Appropriate renal response to hypoperfusion | ### Why This Is Prerenal, Not ATN **Key Point:** The urinary indices in this patient — urine Na⁺ < 20 mEq/L and urine osmolality > 500 mOsm/kg — are the **hallmark of prerenal AKI**, indicating that tubular function is intact. In established ATN, tubular epithelial damage leads to the inability to reabsorb sodium (urine Na⁺ > 40 mEq/L) and inability to concentrate urine (urine osmolality < 350 mOsm/kg, isosthenuria). **Clinical Pearl:** Sepsis causes AKI primarily through hemodynamic mechanisms — reduced cardiac output, vasodilation, and effective volume depletion — leading to renal hypoperfusion. Unless tubular injury has supervened (evidenced by muddy brown casts, epithelial cell casts on urine microscopy, or urine Na⁺ > 40 mEq/L), the AKI is classified as **prerenal**. This patient's urinary indices confirm intact tubular function, making prerenal AKI the correct classification. (Harrison's Principles of Internal Medicine, 21e, Ch. 279) ### Distinguishing Prerenal from Intrinsic Renal (ATN) | Parameter | Prerenal | ATN | |-----------|----------|-----| | Urine Na⁺ | < 20 mEq/L | > 40 mEq/L | | Urine osmolality | > 500 mOsm/kg | < 350 mOsm/kg | | FENa | < 1% | > 2% | | Urine sediment | Normal / hyaline casts | Muddy brown granular casts, epithelial cells | | BUN:Cr ratio | > 20:1 | ~10–15:1 | ### Why Other Options Are Incorrect - **A (ATN):** ATN would show urine Na⁺ > 40 mEq/L and urine osmolality < 350 mOsm/kg due to tubular damage. This patient's indices are opposite. - **B (Postrenal):** No history of obstruction, prostatic disease, or bilateral hydronephrosis; urine output, while low, is not anuric. - **C (Hepatorenal syndrome):** Requires underlying liver disease/cirrhosis with portal hypertension, which is absent here. ### Management - Aggressive fluid resuscitation (crystalloids) guided by hemodynamic response - Vasopressors (norepinephrine) for refractory hypotension - Broad-spectrum antibiotics for sepsis source control - Monitor urine output and creatinine — if AKI does not resolve with resuscitation, reassess for ATN - Avoid nephrotoxins **Mnemonic: PRERENAL** — **P**erfusion reduced, **R**enal tubules intact, **E**lectrolytes conserved (low urine Na⁺), **R**esponse to fluids expected, **E**levated BUN:Cr > 20, **N**o casts on microscopy, **A**DH-mediated concentration preserved, **L**ow urine output [cite: Harrison 21e Ch 279; Brenner & Rector's The Kidney, 11e]