A 52-year-old man with chronic kidney disease (eGFR 25 mL/min/1.73 m²) is prescribed gentamicin 80 mg IV for a urinary tract infection. His serum creatinine is 3.2 mg/dL. After the first dose, peak serum concentration is 8 µg/mL (therapeutic range 4–10 µg/mL). However, the trough concentration 24 hours later is 2.5 µg/mL instead of the expected <1 µg/mL. Which pharmacokinetic parameter is most significantly altered in this patient, and what is the primary consequence?