A 58-year-old man with type 2 diabetes mellitus presents to the emergency department with a 3-day history of vomiting and diarrhea following food poisoning. On examination, he is tachycardic (HR 110/min), hypotensive (BP 88/54 mmHg), and has dry mucous membranes. Laboratory investigations reveal: serum creatinine 3.2 mg/dL (baseline 1.0 mg/dL), BUN 56 mg/dL, urine sodium 8 mEq/L, urine osmolality 580 mOsm/kg, and fractional excretion of sodium (FENa) 0.3%. Urine microscopy shows no casts or cells. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Prerenal AKI ### Key Diagnostic Features **Key Point:** The clinical presentation and urinary findings are classic for prerenal AKI secondary to volume depletion from gastrointestinal losses. ### Why Prerenal AKI? The patient has: 1. **Clinical evidence of hypovolemia**: Tachycardia, hypotension, dry mucous membranes, and 3-day fluid losses 2. **Preserved tubular function**: FENa 0.3% (< 1%), indicating the kidney is avidly reabsorbing sodium in response to perceived volume depletion 3. **Concentrated urine**: Urine osmolality 580 mOsm/kg (> 500), showing appropriate ADH response 4. **Low urine sodium**: 8 mEq/L (< 20), reflecting avid sodium reabsorption 5. **Clean urine microscopy**: No casts, cells, or debris—rules out intrinsic renal disease 6. **Rapid rise in creatinine**: Acute change from baseline 1.0 to 3.2 mg/dL ### Differential Diagnosis Table | Feature | Prerenal AKI | ATN | RPGN | AIN | |---------|-------------|-----|------|-----| | **FENa** | < 1% | > 2% | Variable | > 2% | | **Urine osmolality** | > 500 | < 350 | Variable | Variable | | **Urine sodium** | < 20 | > 40 | Variable | > 40 | | **Urine microscopy** | Clean | Muddy brown casts | RBC casts, dysmorphia | WBC casts, eosinophils | | **Response to fluids** | Rapid improvement | Slow/none | Worsens | Worsens | **High-Yield:** FENa < 1% and urine osmolality > 500 in a patient with acute kidney injury and clinical hypovolemia = **prerenal AKI until proven otherwise**. ### Why Other Options Are Wrong **ATN** would show FENa > 2%, low urine osmolality (< 350), high urine sodium (> 40), and muddy brown casts on microscopy—none of which are present here. **Acute glomerulonephritis** (RPGN) would present with hematuria, RBC casts, and dysmorphic RBCs; the clean urine excludes this. **AIN** typically follows drug exposure (NSAIDs, antibiotics, PPIs) and presents with fever, rash, and eosinophiluria; the clinical context and urinary findings do not fit. ### Management Implications **Clinical Pearl:** Prerenal AKI is **reversible** with aggressive fluid resuscitation. This patient requires: - IV crystalloid bolus (0.9% saline 500–1000 mL over 30–60 min) - Reassessment of volume status and repeat labs in 4–6 hours - Monitoring of urine output and creatinine trend - Correction of underlying cause (antiemetics, electrolyte repletion) **Key Point:** If creatinine does not improve with fluid resuscitation, suspect **intrinsic renal disease** (ATN, RPGN) and escalate investigation (renal biopsy if indicated). [cite:Harrison 21e Ch 279]