Clinical scenario: watery diarrhea | oliguria | raised creatinin
A 3-year-old boy is admitted with a 5-day history of profuse watery diarrhea and vomiting. He received delayed medical attention and was severely dehydrated on arrival. After appropriate fluid resuscitation, his blood pressure normalizes, but 24 hours later he remains oliguric, producing 0.3 mL/kg/hour of urine. Laboratory investigations reveal:
- Serum creatinine: 2.3 mg/dL
- Blood urea nitrogen: 48 mg/dL
- Serum potassium: 6.2 mmol/L
- Urine sodium: 55 mmol/L
- Fractional excretion of sodium (FeNa): 3.5%
- Urine microscopy: numerous muddy brown granular casts
What is the most likely explanation for his persistent acute kidney injury?
A. Ongoing prerenal azotemia due to inadequate fluid replacement
B. Acute tubular necrosis secondary to ischemic renal injury
C. Hemolytic uremic syndrome
D. Acute post-infectious glomerulonephritis
E. Acute interstitial nephritis
Correct answer & Explanation:
correct Answer: B. Acute tubular necrosis secondary to ischemic renal injury
Explanation
This child initially had severe hypovolemia from acute gastroenteritis, resulting in renal ischemia. Although circulation has been restored, he remains oliguric with:
- FeNa >2% (3.5%)
- High urine sodium (>40 mmol/L)
- Muddy brown granular casts
- Persistent rise in serum creatinine despite adequate fluid resuscitation
These findings are classic for ischemic acute tubular necrosis (ATN).
During ischemia, tubular epithelial cells are injured and lose their ability to reabsorb sodium, resulting in high urinary sodium and elevated FeNa. Sloughed tubular epithelial cells form the characteristic muddy brown granular casts.
Why the other options are incorrect
- A. Ongoing prerenal azotemia: In prerenal AKI, the kidney avidly retains sodium, resulting in FeNa <1%, urine sodium <20 mmol/L, and concentrated urine. Renal function usually improves rapidly after fluid resuscitation.
- C. Hemolytic uremic syndrome: Typically follows bloody diarrhea and is characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury, none of which are present here.
- D. Acute post-infectious glomerulonephritis: Usually presents 1–3 weeks after streptococcal infection with hematuria, hypertension, edema, and red cell casts.
- E. Acute interstitial nephritis: Most commonly drug-induced and associated with fever, rash, eosinophilia, and sterile pyuria.
Learning Point
The best discriminator between prerenal AKI and ischemic ATN after severe dehydration is:
| Feature | Prerenal AKI | Acute Tubular Necrosis |
|---|---|---|
| FeNa | <1% | >2% |
| Urine sodium | <20 mmol/L | >40 mmol/L |
| Urine sediment | Bland/hyaline casts | Muddy brown granular casts |
| Response to fluids | Rapid improvement | Persistent AKI despite adequate resuscitation |
Reference:
- KDIGO Clinical Practice Guideline for Acute Kidney Injury (2012; current recommendations)
- Nelson Textbook of Pediatrics, 22nd Edition
- MRCPCH Foundation of Practice & Theory – Pediatric Nephrology principles
