A 68-year-old man with diabetes mellitus type 2 presents with fever, dysuria, and flank pain for 3 days. Urine culture grows Gram-negative rods. Serum creatinine is 1.2 mg/dL (baseline 0.9 mg/dL), and eGFR is 58 mL/min/1.73m². The patient is started on gentamicin 5 mg/kg IV once daily. On day 5 of therapy, serum creatinine rises to 1.8 mg/dL, urine output drops to 400 mL/day, and the patient develops tinnitus. Which of the following best explains the adverse effect observed and represents the most appropriate next step?

Explanation

## Clinical Presentation Analysis The patient exhibits the classic triad of aminoglycoside toxicity: 1. **Nephrotoxicity**: Rising creatinine (1.2 → 1.8 mg/dL), oliguria (400 mL/day), and reduced eGFR 2. **Ototoxicity**: Tinnitus (eighth cranial nerve involvement) 3. **Risk factors present**: Diabetes, baseline renal impairment (eGFR 58), advanced age ## Mechanism of Aminoglycoside-Induced Nephrotoxicity **Key Point:** Aminoglycosides accumulate in proximal tubular cells via endocytosis, generating reactive oxygen species and causing acute tubular necrosis (ATN). This is dose-dependent and partially reversible if caught early. **High-Yield:** The combination of rising creatinine + ototoxicity (tinnitus) in a patient on aminoglycosides is pathognomonic for drug-induced toxicity. Once ototoxicity appears, nephrotoxicity is usually advanced. ## Why Discontinuation is Mandatory | Feature | Aminoglycoside Nephrotoxicity | Aminoglycoside Ototoxicity | |---------|-------------------------------|----------------------------| | **Reversibility** | Partially reversible if drug stopped early | Often irreversible | | **Onset** | Days 3–7 | Days 3–7 | | **Mechanism** | Proximal tubule accumulation → ATN | Cochlear/vestibular hair cell destruction | | **Management** | Discontinue immediately | Discontinue immediately | **Clinical Pearl:** Ototoxicity is a harbinger of severe nephrotoxicity. The presence of tinnitus mandates immediate discontinuation—dose adjustment or therapeutic drug monitoring cannot prevent further damage once symptoms appear. ## Why Switching to Beta-Lactam is Appropriate 1. **Gram-negative coverage**: Cephalosporin (e.g., ceftriaxone) or fluoroquinolone provides adequate coverage for urinary tract infection with Gram-negative organisms 2. **No cross-toxicity**: Beta-lactams have no nephrotoxic or ototoxic potential 3. **Renal function**: Dosing can be adjusted for the patient's reduced eGFR without risk of accumulation-related toxicity **Mnemonic:** **DING** = **D**iscontinue, **I**nterrupt aminoglycoside, **N**ephrotoxicity + ototoxicity, **G**et alternative agent (beta-lactam/fluoroquinolone).