A 72-year-old man with chronic kidney disease stage 3 (baseline creatinine 2.8 mg/dL, eGFR 22 mL/min/1.73m²) presents with fever, dysuria, and suprapubic pain for 3 days. Temperature is 38.6°C, BP 108/72 mmHg, HR 96 bpm. Urinalysis shows pyuria and bacteriuria; urine culture grows Pseudomonas aeruginosa resistant to fluoroquinolones. He is started on gentamicin 5 mg/kg IV daily without renal dosing adjustment. Gentamicin trough levels are not monitored. On day 5 of treatment, serum creatinine increases to 5.1 mg/dL (from baseline 2.8), urine output decreases to 350 mL/24h, and muddy brown urinary casts appear on repeat urinalysis. Which of the following best explains this clinical presentation?

1. A)Concentration-dependent aminoglycoside nephrotoxicity with proximal tubular accumulation and cell deathGABARITO
2. B)Acute allergic interstitial nephritis from gentamicin hypersensitivity reaction
3. C)Hyperkalemia-induced acute tubular necrosis from gentamicin-associated potassium retention
4. D)Prerenal azotemia secondary to gentamicin-induced volume depletion and decreased renal perfusion
5. E)Post-infectious glomerulonephritis with immune complex deposition following Pseudomonas infection