A 58-year-old man with diabetes mellitus type 2 presents with fever, dysuria, and costovertebral angle tenderness. Urine culture grows Escherichia coli. He is started on intravenous gentamicin 5 mg/kg once daily. On day 5 of therapy, his serum creatinine rises from 1.0 mg/dL to 2.8 mg/dL, and urine output drops to 400 mL/day. Urinalysis shows muddy brown casts and proteinuria. Which of the following best explains the nephrotoxicity observed in this patient?

Explanation

## Aminoglycoside Nephrotoxicity Mechanism **Key Point:** Aminoglycoside-induced nephrotoxicity is dose-dependent and primarily affects the proximal tubule, not the glomerulus or interstitium. ### Pathophysiology Gentamicin and other aminoglycosides are filtered by the glomerulus and reabsorbed in the proximal tubule via **megalin-mediated endocytosis**. Once internalized, they accumulate in lysosomes where they inhibit phospholipases and proteases, causing lysosomal dysfunction. This leads to: 1. Disruption of cellular metabolism and mitochondrial function 2. Generation of reactive oxygen species (ROS) 3. Apoptosis and necrosis of tubular epithelial cells 4. **Acute tubular necrosis (ATN)** with muddy brown casts and proteinuria **High-Yield:** The clinical presentation—rising creatinine by day 5, muddy brown casts, and non-oliguric renal failure—is pathognomonic for aminoglycoside-induced ATN, not interstitial nephritis or prerenal disease. ### Risk Factors in This Patient - **Diabetes mellitus** (impaired renal perfusion and glomerular filtration) - **Age ≥50 years** - **Prolonged therapy** (day 5 of gentamicin) - **Elevated baseline creatinine** (even mild elevation increases risk) **Clinical Pearl:** Once-daily dosing (as given here: 5 mg/kg once daily) is less nephrotoxic than divided dosing because it allows for a "drug-free interval" permitting tubular cell recovery. However, nephrotoxicity can still occur, especially in high-risk patients like this diabetic man. ### Distinguishing Features | Feature | Aminoglycoside Nephrotoxicity | Interstitial Nephritis | Prerenal Azotemia | |---------|-------------------------------|------------------------|-------------------| | **Timing** | Days 5–7 of therapy | Days 1–3 (hypersensitivity) | Acute, reversible | | **Urine sediment** | Muddy brown casts, proteinuria | WBC casts, eosinophiluria | Hyaline casts, concentrated | | **BUN/Cr ratio** | <10 (intrinsic renal disease) | <10 | >20 (prerenal) | | **Urine osmolality** | Low (tubular damage) | Variable | High (>500 mOsm/kg) | | **Response to fluids** | No improvement | No improvement | Rapid improvement | **Mnemonic for aminoglycoside toxicity: "OTOTOXIC NEPHRO"** - **O**tocity (8th nerve damage → hearing loss, vertigo) - **T**ubular necrosis (proximal tubule ATN) - **O**xidative stress (ROS generation) - **T**oxin accumulation (lysosomes) - **O**smotic imbalance - **X**enobiotic metabolism impairment - **I**ncreased risk with age, renal disease, dehydration - **C**umulative dose effect **Warning:** Do not confuse aminoglycoside nephrotoxicity with acute interstitial nephritis (AIN), which typically presents with fever, rash, and eosinophiluria within 1–3 days of drug exposure and is immune-mediated, not dose-dependent.